MINOR: mux-h2: Forward clients scheme to servers checking start-line flags

MINOR: mux-h2: Forward clients scheme to servers checking start-line flags

By default, the scheme "https" is always used. But when an explicit scheme was
defined and when this scheme is "http", we use it in the request sent to the
server. This is done by checking flags of the start-line. If the flag
HTX_SL_F_HAS_SCHM is set, it means an explicit scheme was defined on the client
side. And if the flag HTX_SL_F_SCHM_HTTP is set, it means the scheme "http" was
used.