[release-branch.go1.9] net/http: update bundled http2

Updates http2 to x/net/http2 git rev 44b7c21 for

  http2: Discard data reads on HEAD requests
  https://golang.org/cl/88655

Fixes golang/go#22376

Change-Id: I931d9065d7309bc6d3f978bfe8cc6a9f940ce9e9
Reviewed-on: https://go-review.googlesource.com/88676


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>