[5.5] Allow to fetch specific key when using json helper (#22489)

* allow to fetch specific key when using json helper

* fix docblocks

src/Illuminate/Foundation/Testing/TestResponse.php

@@ -500,9 +500,10 @@ public function assertJsonValidationErrors($keys)
     /**
      * Validate and return the decoded response JSON.
      *
-     * @return array
+     * @param  string|null  $key
+     * @return mixed
      */
-    public function decodeResponseJson()
+    public function decodeResponseJson($key = null)
     {
         $decodedResponse = json_decode($this->getContent(), true);
 
@@ -514,17 +515,18 @@ public function decodeResponseJson()
             }
         }
 
-        return $decodedResponse;
+        return data_get($decodedResponse, $key);
     }
 
     /**
      * Validate and return the decoded response JSON.
      *
-     * @return array
+     * @param  string|null  $key
+     * @return mixed
      */
-    public function json()
+    public function json($key = null)
     {
-        return $this->decodeResponseJson();
+        return $this->decodeResponseJson($key);
     }
 
     /**

tests/Foundation/FoundationTestResponseTest.php

@@ -158,6 +158,17 @@ public function testCanBeCreatedFromBinaryFileResponses()
 
         $files->deleteDirectory($tempDir);
     }
+
+    public function testJsonHelper()
+    {
+        $response = TestResponse::fromBaseResponse(new Response(new JsonSerializableMixedResourcesStub));
+
+        $this->assertEquals('foo', $response->json('foobar.foobar_foo'));
+        $this->assertEquals(
+            json_decode($response->getContent(), true),
+            $response->json()
+        );
+    }
 }
 
 class JsonSerializableMixedResourcesStub implements JsonSerializable