update symfony vendor, fix response conversion

src/Silex/Framework.php

@@ -310,8 +310,12 @@ class Framework extends HttpKernel
      */
     public function parseStringResponse(Event $event, $response)
     {
+        if ($response instanceof Response) {
+            return $response;
+        } else {
             return new Response((string) $response);
         }
+    }
 
     /**
      * Handler for core.view

symfony @ a5bac4b5

-Subproject commit a72145bf5f72ca9568262425df0fa3942738af93
+Subproject commit a5bac4b5a3036d80d68c850a858ed08fad41a918