add 421 status_code

[busla]

Would be great to see http 421 status_code (Misdirected request) added to https://github.com/pallets/werkzeug/blob/master/werkzeug/http.py#L142, to return an informative response when Flask tries to communicate as a proxy to external sources, e.g. authentication services, geolookup, et.c.

Cheers!