Fixed broken ResponseFactory

Broken against zend-servicemanager v2, as it was using constant notation
instead of a string when invoking itself.

Author: weierophinney

src/Service/ResponseFactory.php

@@ -39,6 +39,6 @@ public function __invoke(ContainerInterface $container, $name, array $options =
      */
     public function createService(ServiceLocatorInterface $container)
     {
-        return $this($container, HttpResponse);
+        return $this($container, 'Response');
     }
 }