Check if SymfonyHttplug actually implements HttpClient (#253)

* Check if SymfonyHttplug actually implements HttpClient

* Use is_subclass_of

Co-authored-by: Nicolas Grekas <nicolas.grekas@gmail.com>

---------

Co-authored-by: Nicolas Grekas <nicolas.grekas@gmail.com>

src/Strategy/CommonClassesStrategy.php

@@ -78,7 +78,7 @@ final class CommonClassesStrategy implements DiscoveryStrategy
             ['class' => React::class, 'condition' => React::class],
         ],
         HttpClient::class => [
-            ['class' => SymfonyHttplug::class, 'condition' => [SymfonyHttplug::class, [self::class, 'isPsr17FactoryInstalled']]],
+            ['class' => SymfonyHttplug::class, 'condition' => [SymfonyHttplug::class, [self::class, 'isPsr17FactoryInstalled'], [self::class, 'isSymfonyImplementingHttpClient']]],
             ['class' => Guzzle7::class, 'condition' => Guzzle7::class],
             ['class' => Guzzle6::class, 'condition' => Guzzle6::class],
             ['class' => Guzzle5::class, 'condition' => Guzzle5::class],
@@ -158,6 +158,11 @@ public static function isGuzzleImplementingPsr18()
         return defined('GuzzleHttp\ClientInterface::MAJOR_VERSION');
     }
 
+    public static function isSymfonyImplementingHttpClient()
+    {
+        return is_subclass_of(SymfonyHttplug::class, HttpClient::class);
+    }
+
     /**
      * Can be used as a condition.
      *