MINOR: hpack: provide new functions to encode the ":status" header

MINOR: hpack: provide new functions to encode the ":status" header

This header exists with 7 different values, it's worth taking them
into account for the encoding, hence these functions. One of them
makes use of an integer only and computes the 3 output bytes in case
of literal. The other one benefits from the knowledge of an existing
string, which for example exists in the case of H1 to H2 encoding.