Updates to git rev 042ba42f (https://golang.org/cl/16734)

This moves all the code for glueing the HTTP1 and HTTP2 transports
together out of net/http and into x/net/http2 where others can use it,
and where it has tests.

Change-Id: I143ac8bb61eed36c87fd838b682ebb37b81b8c2c
Reviewed-on: https://go-review.googlesource.com/16735
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>