Updates x/net/http2 to golang.org/cl/23220
(http2: with Go 1.7 set Request.Context in ServeHTTP handlers)

Fixes #15134

Change-Id: I73bac2601118614528f051e85dab51dc48e74f41
Reviewed-on: https://go-review.googlesource.com/23221
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>