return value for __invoke was wrong

Corrected  __invoke return value.

tests/test_app/TestApp/Http/BadResponseApplication.php

@@ -28,6 +28,6 @@ class BadResponseApplication extends BaseApplication
      */
     public function __invoke(ServerRequestInterface $request, ResponseInterface $response, $next)
     {
-        return $res;
+        return $response;
     }
 }