[HttpFoundation] Restore compliance with RFC2324

Author: Drak

src/Symfony/Component/HttpFoundation/Response.php

@@ -101,6 +101,7 @@ class Response
  415 => 'Unsupported Media Type',
  416 => 'Requested Range Not Satisfiable',
  417 => 'Expectation Failed',
+ 418 => 'I\'m a teapot',
  422 => 'Unprocessable Entity', // RFC4918
  423 => 'Locked', // RFC4918
  424 => 'Failed Dependency', // RFC4918