MINOR: h2: implement H2->HTX request header frame transcoding

MINOR: h2: implement H2->HTX request header frame transcoding

Till now we could only produce an HTTP/1 request from a list of H2
request headers. Now the new function h2_make_htx_request() does the
same but using the HTX encoding instead, while respecting the H2
semantics. The code is not much different from the first version,
only the encoding differs.

For now it's not used.