Updates http2 to x/net/http2 git rev 973f3f3 for:

   http2: make Transport treat http.NoBody like it were nil
   https://golang.org/cl/45993

Updates #18891

Change-Id: I846ccf286992ed2c6249014e51fdeb40b35e50ed
Reviewed-on: https://go-review.googlesource.com/46000
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>