MEDIUM: h2: make the request parser rebuild a complete URI

MEDIUM: h2: make the request parser rebuild a complete URI

Till now we've been producing path components of the URI and using the
:authority header only to be placed into the host part. But this practice
is not correct, as if we're used to convey H1 proxy requests over H2 then
over H1, the absolute URI is presented as a path on output, which is not
valid. In addition the scheme on output is not updated from the absolute
URI either.

Now the request parser will continue to deliver origin-form for request
received using the http/https schemes, but will use the absolute-form
when dealing with other schemes, by concatenating the scheme, the authority
and the path if it's not '*'.