MINOR: mux-h1: Add flags if outgoing msg contains a header about its payload

MINOR: mux-h1: Add flags if outgoing msg contains a header about its payload

If a "Content-length" or "Transfer-Encoding; chunked" headers is found or
inserted in an outgoing message, a specific flag is now set on the H1
stream. H1S_F_HAVE_CLEN is set for "Content-length" header and
H1S_F_HAVE_CHNK for "Transfer-Encoding: chunked".

This will be useful to properly format outgoing messages, even if one of
these headers was removed by hand (with no update of the message meta-data).