Add support for HTTP 451 (RFC 7725)

See [rfc7725 section 3](https://tools.ietf.org/html/rfc7725#section-3).

src/cow_http.erl

@@ -321,6 +321,7 @@ status(426) -> <<"426 Upgrade Required">>;
 status(428) -> <<"428 Precondition Required">>;
 status(429) -> <<"429 Too Many Requests">>;
 status(431) -> <<"431 Request Header Fields Too Large">>;
+status(451) -> <<"451 Unavailable For Legal Reasons">>;
 status(500) -> <<"500 Internal Server Error">>;
 status(501) -> <<"501 Not Implemented">>;
 status(502) -> <<"502 Bad Gateway">>;