This adds the http2 Transport method needed by
https://golang.org/cl/16090 to glue the HTTP/1 Transport
(net/http.Transport) to the http2.Transport without throwing away
fresh TCP connections after the TLS handshake determines which
NPN/ALPN protocol was selected.

Updates golang/go#6891

Change-Id: Iba004c8b1a149a5ee6c755d9a3fc1b19856a4e47
Reviewed-on: https://go-review.googlesource.com/16180Reviewed-by: Andrew Gerrand <adg@golang.org>