Support PSR-18 interface as candidate by inheritance (#145)

* Support PSR-18 interface as candidate by inheritance

spec/Strategy/MockClientStrategySpec.php

@@ -8,6 +8,7 @@
 use Http\Discovery\Strategy\DiscoveryStrategy;
 use Http\Discovery\Strategy;
 use PhpSpec\ObjectBehavior;
+use Psr\Http\Client\ClientInterface;
 use spec\Http\Discovery\Helper\DiscoveryHelper;
 
 class MockClientStrategySpec extends ObjectBehavior
@@ -24,6 +25,15 @@ function it_should_return_the_mock_client(DiscoveryStrategy $strategy)
         $candidates->shouldHaveCount(1);
     }
 
+    function it_should_return_the_mock_client_for_implementations(DiscoveryStrategy $strategy)
+    {
+        foreach (class_implements(HttpClient::class) as $type) {
+            $candidates = $this->getCandidates($type);
+            $candidates->shouldBeArray();
+            $candidates->shouldHaveCount(1);
+        }
+    }
+
     function it_should_return_the_mock_client_as_async(DiscoveryStrategy $strategy)
     {
         $candidates = $this->getCandidates(HttpAsyncClient::class);

src/Strategy/MockClientStrategy.php

@@ -18,12 +18,10 @@ final class MockClientStrategy implements DiscoveryStrategy
      */
     public static function getCandidates($type)
     {
-        switch ($type) {
-            case HttpClient::class:
-            case HttpAsyncClient::class:
-                return [['class' => Mock::class, 'condition' => Mock::class]];
-            default:
-                return [];
-       }
+        if (is_a(HttpClient::class, $type, true) || is_a(HttpAsyncClient::class, $type, true)) {
+            return [['class' => Mock::class, 'condition' => Mock::class]];
+        }
+
+        return [];
     }
 }