MINOR: h2/htx: Set the flag HTX_SL_F_BODYLESS for messages without body

MINOR: h2/htx: Set the flag HTX_SL_F_BODYLESS for messages without body

This information is usefull to know if a body is expected or not, regardless the
presence or not of the header "Content-Length" and its value. Once the ES flag
is set on the header frame or when the content length is 0, we can safely add
the flag HTX_SL_F_BODYLESS on the HTX start-line.

Among other things, it will help the mux-h1 to know if it should add TE header
or not. It will also help the HTTP compression filter.

This patch must be backported to 1.9 because a bug fix depends on it.