http2: fix all vet warnings

Updates golang/go#16228
Updates golang/go#11041

Change-Id: I2b50c2f4bfaae2d9ad59bc78e1c7c3e807f08075
Reviewed-on: https://go-review.googlesource.com/28344
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
6 files changed