MINOR: h2: detect presence of CONNECT and/or content-length

MINOR: h2: detect presence of CONNECT and/or content-length

We'll need this in order to support uploading chunks. The h2 to h1
converter checks for the presence of the content-length header field
as well as the CONNECT method and returns these information to the
caller. The caller indicates whether or not a body is detected for
the message (presence of END_STREAM or not). No transfer-encoding
header is emitted yet.