added missing constant in Response

Author: fabpot

src/Symfony/Component/HttpFoundation/Response.php

@@ -59,6 +59,7 @@ class Response
     const HTTP_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
     const HTTP_EXPECTATION_FAILED = 417;
     const HTTP_I_AM_A_TEAPOT = 418;                                               // RFC2324
+    const HTTP_MISDIRECTED_REQUEST = 421;                                         // RFC7540
     const HTTP_UNPROCESSABLE_ENTITY = 422;                                        // RFC4918
     const HTTP_LOCKED = 423;                                                      // RFC4918
     const HTTP_FAILED_DEPENDENCY = 424;                                           // RFC4918