diff --git a/core/core.api.php b/core/core.api.php
index 63fd607..9f1d4f8 100644
--- a/core/core.api.php
+++ b/core/core.api.php
@@ -2190,6 +2190,9 @@ function hook_validation_constraint_alter(array &$definitions) {
  * - context_provider: Indicates a block context provider, used for example
  *   by block conditions. It has to implement
  *   \Drupal\Core\Plugin\Context\ContextProviderInterface.
+ * - http_client_middleware: Indicates that the service provides a guzzle
+ *   middleware, see https://guzzle.readthedocs.org/en/latest/handlers-and-middleware.html
+ *   for more information.
  *
  * Creating a tag for a service does not do anything on its own, but tags
  * can be discovered or queried in a compiler pass when the container is built,
diff --git a/core/lib/Drupal.php b/core/lib/Drupal.php
index 9ff3861..baebc75 100644
--- a/core/lib/Drupal.php
+++ b/core/lib/Drupal.php
@@ -403,6 +403,8 @@ public static function httpClient() {
   }
 
   /**
+   * Returns a factory for guzzle HTTP clients with Drupal drupal options.
+   *
    * @return \Drupal\Core\Http\ClientFactory
    */
   public static function httpClientFactory() {
diff --git a/core/lib/Drupal/Core/DependencyInjection/Compiler/GuzzleMiddlewarePass.php b/core/lib/Drupal/Core/DependencyInjection/Compiler/GuzzleMiddlewarePass.php
index d2523cf..42e403c 100644
--- a/core/lib/Drupal/Core/DependencyInjection/Compiler/GuzzleMiddlewarePass.php
+++ b/core/lib/Drupal/Core/DependencyInjection/Compiler/GuzzleMiddlewarePass.php
@@ -9,7 +9,6 @@
 
 use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
-use Symfony\Component\DependencyInjection\Reference;
 
 class GuzzleMiddlewarePass implements CompilerPassInterface {
 
diff --git a/core/lib/Drupal/Core/Http/ClientFactory.php b/core/lib/Drupal/Core/Http/ClientFactory.php
index b9d597a..f6471e5 100644
--- a/core/lib/Drupal/Core/Http/ClientFactory.php
+++ b/core/lib/Drupal/Core/Http/ClientFactory.php
@@ -44,7 +44,7 @@ public function __construct(HandlerStack $stack) {
    *   The HTTP client.
    */
   public function fromOptions(array $config = []) {
-    $default_config = array(
+    $default_config = [
       // Security consideration: we must not use the certificate authority
       // file shipped with Guzzle because it can easily get outdated if a
       // certificate authority is hacked. Instead, we rely on the certificate
@@ -53,16 +53,16 @@ public function fromOptions(array $config = []) {
       // path to the pem file bundled with Guzzle.
       'verify' => TRUE,
       'timeout' => 30,
-      'headers' => array(
+      'headers' => [
         'User-Agent' => 'Drupal/' . \Drupal::VERSION . ' (+https://www.drupal.org/) ' . \GuzzleHttp\default_user_agent(),
-      ),
+      ],
       'handler' => $this->stack,
-    );
+    ];
 
     // The entire config array is merged/configurable to allow Guzzle client
     // options outside of 'defaults' to be changed, such as 'adapter', or
     // 'message_factory'.
-    $config = NestedArray::mergeDeep($config, $default_config, Settings::get('http_client_config', array()));
+    $config = NestedArray::mergeDeep($config, $default_config, Settings::get('http_client_config', []));
 
     return new Client($config);
   }
diff --git a/core/lib/Drupal/Core/Http/HandlerStackConfigurator.php b/core/lib/Drupal/Core/Http/HandlerStackConfigurator.php
index 3dfc6b6..8e9e36a 100644
--- a/core/lib/Drupal/Core/Http/HandlerStackConfigurator.php
+++ b/core/lib/Drupal/Core/Http/HandlerStackConfigurator.php
@@ -19,6 +19,8 @@
  * as http_client_middleware. Each service must contain an __invoke method that
  * returns a closure which will serve as the middleware.
  *
+ * @see https://guzzle.readthedocs.org/en/latest/handlers-and-middleware.html
+ *
  * @see \Drupal\Core\Http\Client
  * @see \Drupal\Core\Test\EventSubscriber\HttpRequestSubscriber
  */
@@ -66,9 +68,12 @@ protected function initializeMiddlewares() {
       $this->middlewares = [];
       foreach ($this->middlewareIds as $middleware_id) {
         $middleware = $this->container->get($middleware_id);
-        if (method_exists($middleware, '__invoke')) {
+        if (is_callable($middleware)) {
           $this->middlewares[$middleware_id] = $middleware();
         }
+        else {
+          throw new \InvalidArgumentException('Middlewares need to implement __invoke, see https://guzzle.readthedocs.org/en/latest/handlers-and-middleware.html for more information about middlewares.');
+        }
       }
     }
   }
diff --git a/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php b/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php
index e3d2504..1f8f576 100644
--- a/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php
+++ b/core/lib/Drupal/Core/Test/EventSubscriber/HttpRequestSubscriber.php
@@ -8,7 +8,6 @@
 namespace Drupal\Core\Test\EventSubscriber;
 
 use Drupal\Core\Http\StackHandlerRegistrationInterface;
-use GuzzleHttp\HandlerStack;
 use Psr\Http\Message\RequestInterface;
 
 /**
diff --git a/core/modules/statistics/src/Tests/StatisticsAdminTest.php b/core/modules/statistics/src/Tests/StatisticsAdminTest.php
index cdd8a48..9762d0a 100644
--- a/core/modules/statistics/src/Tests/StatisticsAdminTest.php
+++ b/core/modules/statistics/src/Tests/StatisticsAdminTest.php
@@ -40,7 +40,7 @@ class StatisticsAdminTest extends WebTestBase {
   /**
    * The Guzzle HTTP client.
    *
-   * @var \GuzzleHttp\ClientInterface;
+   * @var \GuzzleHttp\Client;
    */
   protected $client;
 
@@ -78,16 +78,16 @@ function testStatisticsSettings() {
     $post = array('nid' => $nid);
     global $base_url;
     $stats_path = $base_url . '/' . drupal_get_path('module', 'statistics'). '/statistics.php';
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
 
     // Hit the node again (the counter is incremented after the hit, so
     // "1 view" will actually be shown when the node is hit the second time).
     $this->drupalGet('node/' . $this->testNode->id());
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
     $this->assertText('1 view', 'Node is viewed once.');
 
     $this->drupalGet('node/' . $this->testNode->id());
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
     $this->assertText('2 views', 'Node is viewed 2 times.');
   }
 
@@ -103,7 +103,7 @@ function testDeleteNode() {
     $post = array('nid' => $nid);
     global $base_url;
     $stats_path = $base_url . '/' . drupal_get_path('module', 'statistics'). '/statistics.php';
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
 
     $result = db_select('node_counter', 'n')
       ->fields('n', array('nid'))
@@ -137,9 +137,9 @@ function testExpiredLogs() {
     $post = array('nid' => $nid);
     global $base_url;
     $stats_path = $base_url . '/' . drupal_get_path('module', 'statistics'). '/statistics.php';
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
     $this->drupalGet('node/' . $this->testNode->id());
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
     $this->assertText('1 view', 'Node is viewed once.');
 
     // statistics_cron() will subtract
diff --git a/core/modules/statistics/src/Tests/StatisticsLoggingTest.php b/core/modules/statistics/src/Tests/StatisticsLoggingTest.php
index ce58e0e..c26ea04 100644
--- a/core/modules/statistics/src/Tests/StatisticsLoggingTest.php
+++ b/core/modules/statistics/src/Tests/StatisticsLoggingTest.php
@@ -36,7 +36,7 @@ class StatisticsLoggingTest extends WebTestBase {
   /**
    * The Guzzle HTTP client.
    *
-   * @var \GuzzleHttp\ClientInterface;
+   * @var \GuzzleHttp\Client;
    */
   protected $client;
 
@@ -94,7 +94,7 @@ function testLogging() {
     // Manually call statistics.php to simulate ajax data collection behavior.
     $nid = $this->node->id();
     $post = array('nid' => $nid);
-    $this->client->post($stats_path, array('body' => $post));
+    $this->client->post($stats_path, array('form_params' => $post));
     $node_counter = statistics_get($this->node->id());
     $this->assertIdentical($node_counter['totalcount'], '1');
   }
diff --git a/core/modules/statistics/src/Tests/Views/IntegrationTest.php b/core/modules/statistics/src/Tests/Views/IntegrationTest.php
index d83a387..a186779 100644
--- a/core/modules/statistics/src/Tests/Views/IntegrationTest.php
+++ b/core/modules/statistics/src/Tests/Views/IntegrationTest.php
@@ -76,9 +76,8 @@ public function testNodeCounterIntegration() {
     // @see \Drupal\statistics\Tests\StatisticsLoggingTest::testLogging().
     global $base_url;
     $stats_path = $base_url . '/' . drupal_get_path('module', 'statistics'). '/statistics.php';
-    $client = \Drupal::httpClient();
-    $client->setDefaultOption('config/curl', array(CURLOPT_TIMEOUT => 10));
-    $client->post($stats_path, array('body' => array('nid' => $this->node->id())));
+    $client = \Drupal::httpClientFactory()->fromOptions(['config/curl', array(CURLOPT_TIMEOUT => 10)]);
+    $client->post($stats_path, array('form_params' => array('nid' => $this->node->id())));
     $this->drupalGet('test_statistics_integration');
 
     $expected = statistics_get($this->node->id());