http2: never Read from Request.Body in Transport to determine ContentLength

Updates golang/go#17480 (fixes after vendor to std)
Updates golang/go#17071 (a better fix than the original one)

Change-Id: Ibb49d2cb825e28518e688b5c3e0952956a305050
Reviewed-on: https://go-review.googlesource.com/31326
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
2 files changed