Updates to golang.org/x/net/http2 git rev 28273ec9 for
https://golang.org/cl/17937

Fixes #13648

Change-Id: I27c77524b2e4a172c5f8be08f6fbb0f2e2e4b200
Reviewed-on: https://go-review.googlesource.com/17938Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>