http2: Deal with HTTP/2 data inside response header buffer

Previously if HTTP/2 traffic is appended to HTTP Upgrade response header
(thus they are in the same buffer), the trailing HTTP/2 traffic is not
processed and lost.  The appended data is most likely SETTINGS frame.
If it is lost, nghttp2 library complains server does not obey the HTTP/2
protocol and issues GOAWAY frame and curl eventually drops connection.
This commit fixes this problem and now trailing data is processed.