MEDIUM: mux-h2: do not map Host to :authority on output

MEDIUM: mux-h2: do not map Host to :authority on output

Instead of mapping the Host header field to :authority, we now act
differently if the request is in origin form or in absolute form.
If it's absolute, we extract the scheme and the authority from the
request, fix the path if it's empty, and drop the Host header.
Otherwise we take the scheme from the http/https flags in the HTX
layer, make the URI be the path only, and emit the Host header,
as indicated in RFC7540#8.1.2.3. This allows to distinguish between
absolute and origin requests for H1 to H2 conversions.