MINOR: h3: complete HTTP/3 request scheme encoding

MINOR: h3: complete HTTP/3 request scheme encoding

Previously, scheme was always set to https when transcoding an HTX
start-line into a HTTP/3 request. Change this so this conversion is now
fully compliant.

If no scheme is specified by the client, which is what happens most of
the time with HTTP/1, https is set for the HTTP/3 request. Else, reuse
the scheme requested by the client.

If either https or http is set, qpack_encode_scheme will encode it using
entry from QPACK static table. Else, a full literal field line with name
ref is used instead as the scheme value is specified as-is.