Update Response.php

Hello,

I just added a missing comment for HTTP 451 status code to the associated RFC.

Author: abdounikarim

Response.php

@@ -77,7 +77,7 @@ class Response
     public const HTTP_PRECONDITION_REQUIRED = 428;                                       // RFC6585
     public const HTTP_TOO_MANY_REQUESTS = 429;                                           // RFC6585
     public const HTTP_REQUEST_HEADER_FIELDS_TOO_LARGE = 431;                             // RFC6585
-    public const HTTP_UNAVAILABLE_FOR_LEGAL_REASONS = 451;
+    public const HTTP_UNAVAILABLE_FOR_LEGAL_REASONS = 451;                               // RFC7725
     public const HTTP_INTERNAL_SERVER_ERROR = 500;
     public const HTTP_NOT_IMPLEMENTED = 501;
     public const HTTP_BAD_GATEWAY = 502;