MINOR: h2: Set flags about the request's scheme on the start-line

MINOR: h2: Set flags about the request's scheme on the start-line

The flag HTX_SL_F_HAS_SCHM is always set because H2 requests have always an
explicit scheme. Then, the pseudo-header ":scheme" is tested. If it is set to
"http", the flag HTX_SL_F_SCHM_HTTP is set. Otherwise, for all other cases, the
flag HTX_SL_F_SCHM_HTTPS is set. For now, it seems reasonable to have a fallback
on the scheme "https".