diff --git a/core/lib/Drupal/Core/Http/Client.php b/core/lib/Drupal/Core/Http/Client.php
index feab56a..60b7b23 100644
--- a/core/lib/Drupal/Core/Http/Client.php
+++ b/core/lib/Drupal/Core/Http/Client.php
@@ -7,8 +7,9 @@
 
 namespace Drupal\Core\Http;
 
-use GuzzleHttp\Client as GuzzleClient;
+use Drupal\Core\Http\Plugin\SimpletestHttpRequestSubscriber;
 use Drupal\Component\Utility\NestedArray;
+use GuzzleHttp\Client as GuzzleClient;
 
 /**
  * Drupal default HTTP client class.
@@ -19,28 +20,11 @@ class Client extends GuzzleClient {
    * {@inheritdoc}
    */
   public function __construct(array $config = []) {
-    // If the database prefix is being used by SimpleTest to run the tests in a
-    // copied database then set the user-agent header to the database prefix so
-    // that any calls to other Drupal pages will run the SimpleTest prefixed
-    // database. The user-agent is used to ensure that multiple testing sessions
-    // running at the same time won't interfere with each other as they would if
-    // the database prefix were stored statically in a file or database
-    // variable.
-    if ($test_prefix = drupal_valid_test_ua()) {
-      $default_user_agent = drupal_generate_test_ua($test_prefix);
-    }
-    else {
-      $default_user_agent = 'Drupal (+http://drupal.org/)';
-    }
-
     $defaults = array(
-      'curl' => array(
-        CURLOPT_TIMEOUT => 30,
-        CURLOPT_MAXREDIRS => 3,
-      ),
-      'defaults' => array(
-        'headers' => array(
-          'User-Agent' => $default_user_agent,
+      'config' => array(
+        'curl' => array(
+          CURLOPT_TIMEOUT => 30,
+          CURLOPT_MAXREDIRS => 3,
         ),
         // Security consideration: we must not use the certificate authority file
         // shipped with Guzzle because it can easily get outdated if a certificate
@@ -51,10 +35,17 @@ public function __construct(array $config = []) {
           'certificate_authority' => 'system',
         ),
       ),
+      'defaults' => array(
+        'headers' => array(
+          'User-Agent' => 'Drupal (+http://drupal.org/)',
+        ),
+      ),
     );
     $config = NestedArray::mergeDeep($defaults, $config);
 
     parent::__construct($config);
+
+    $this->getEmitter()->attach(new SimpletestHttpRequestSubscriber());
   }
 
 
diff --git a/core/lib/Drupal/Core/Http/Plugin/SimpletestHttpRequestSubscriber.php b/core/lib/Drupal/Core/Http/Plugin/SimpletestHttpRequestSubscriber.php
new file mode 100644
index 0000000..825588c
--- /dev/null
+++ b/core/lib/Drupal/Core/Http/Plugin/SimpletestHttpRequestSubscriber.php
@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\Core\Http\Plugin\SimpletestHttpRequestSubscriber
+ */
+
+namespace Drupal\Core\Http\Plugin;
+
+use GuzzleHttp\Event\BeforeEvent;
+use GuzzleHttp\Event\SubscriberInterface;
+
+/**
+ * Subscribe to HTTP requests and override the User-Agent header if the request
+ * is being dispatched from inside a simpletest.
+ */
+class SimpletestHttpRequestSubscriber implements SubscriberInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getEvents() {
+    return array(
+      'before'   => array('onBeforeSendRequest'),
+    );
+  }
+
+  /**
+   * Event callback for the 'before' event
+   */
+  public function onBeforeSendRequest(BeforeEvent $event) {
+    // If the database prefix is being used by SimpleTest to run the tests in a copied
+    // database then set the user-agent header to the database prefix so that any
+    // calls to other Drupal pages will run the SimpleTest prefixed database. The
+    // user-agent is used to ensure that multiple testing sessions running at the
+    // same time won't interfere with each other as they would if the database
+    // prefix were stored statically in a file or database variable.
+    if ($test_prefix = drupal_valid_test_ua()) {
+      $event->getRequest()->setHeader('User-Agent', drupal_generate_test_ua($test_prefix));
+    }
+  }
+}