Issue #2251829 by damiankloip: Add 'Guzzle' back to HTTP clients default user agent header.

core/lib/Drupal/Core/Http/Client.php

@@ -31,7 +31,7 @@ public function __construct(array $config = []) {
       'verify' => TRUE,
       'timeout' => 30,
       'headers' => array(
-        'User-Agent' => 'Drupal (+http://drupal.org/)',
+        'User-Agent' => 'Drupal/' . \Drupal::VERSION . ' (+https://drupal.org/) ' . static::getDefaultUserAgent(),
       ),
     );