diff --git a/core/core.services.yml b/core/core.services.yml
index 080e629..ea0afdc 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -152,12 +152,6 @@ services:
     arguments: ['@path.alias_storage', '@path.alias_whitelist', '@language_manager']
   http_default_client:
     class: Drupal\Core\Http\Client
-    # 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-    # shipped with Guzzle because it can easily get outdated if a certificate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 authority file
-    # provided by the operating system which is more likely going to be updated
-    # in a timely fashion.
-    arguments: [{config: {curl : {CURLOPT_TIMEOUT: 30, CURLOPT_MAXREDIRS: 3}, ssl: {ssl.certificate_authority: system}}}]
   theme.negotiator:
     class: Drupal\Core\Theme\ThemeNegotiator
     arguments: ['@access_check.theme', '@request_stack']
diff --git a/core/lib/Drupal/Core/Http/Client.php b/core/lib/Drupal/Core/Http/Client.php
index 555672a..953e7a4 100644
--- a/core/lib/Drupal/Core/Http/Client.php
+++ b/core/lib/Drupal/Core/Http/Client.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Http;
 
 use GuzzleHttp\Client as GuzzleClient;
+use Drupal\Component\Utility\NestedArray;
 
 /**
  * Drupal default HTTP client class.
@@ -18,8 +19,6 @@ class Client extends GuzzleClient {
    * {@inheritdoc}
    */
   public function __construct(array $config = []) {
-    parent::__construct($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
@@ -34,7 +33,26 @@ public function __construct(array $config = []) {
       $default_user_agent = 'Drupal (+http://drupal.org/)';
     }
 
-    $this->setDefaultOption('headers/User-Agent', $default_user_agent);
+    $defaults = array(
+      'curl' => array(
+        CURLOPT_TIMEOUT => 30,
+        CURLOPT_MAXREDIRS => 3,
+      ),
+      'headers' => array(
+        'User-Agent' => $default_user_agent,
+      ),
+      // 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+      // shipped with Guzzle because it can easily get outdated if a certificate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 authority file
+      // provided by the operating system which is more likely going to be
+      // updated in a timely fashion.
+      'ssl' => array(
+        'certificate_authority' => 'system',
+      ),
+    );
+    $config = NestedArray::mergeDeep($defaults, $config);
+
+    parent::__construct($config);
   }
 
 
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/fetcher/DefaultFetcher.php b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/fetcher/DefaultFetcher.php
index d839df1..bd6b8ed 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/fetcher/DefaultFetcher.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/fetcher/DefaultFetcher.php
@@ -78,8 +78,8 @@ public function fetch(FeedInterface $feed) {
       }
 
       $feed->source_string = $response->getBody(TRUE);
-      $feed->setEtag($response->getEtag());
-      $feed->setLastModified(strtotime($response->getLastModified()));
+      $feed->setEtag($response->getHeader('ETag'));
+      $feed->setLastModified(strtotime($response->getHeader('Last-Modified')));
       $feed->http_headers = $response->getHeaders();
 
       // Update the feed URL in case of a 301 redirect.