http2: with Go 1.7 set Request.Context in ServeHTTP handlers

And act the same as HTTP/1.x in Go 1.7.

Updates golang/go#15134

Change-Id: Ib64dd82cc5f8dd60e1680525f664d5b72be11fc6
Reviewed-on: https://go-review.googlesource.com/23220
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>