Add status

Add 102, 207, 208, 422, 423, 424, 426, 429, 431, 506, 507, 508, 510 and 511.

Asking a review from @hoaproject/hoackers :-).
It seems this does not exist anymore. BTW, I found [texts talking about it](https://tools.ietf.org/html/draft-cohen-http-305-306-responses-00), they refer to `306` as `Switch Proxy`