net/http: update bundled http2

Updates x/net/http2 to git rev 6a513af for:

  http2: return flow control for closed streams
  https://golang.org/cl/25231

  http2: make Transport prefer HTTP response header recv before body write error
  https://golang.org/cl/24984

  http2: make Transport treat "Connection: close" the same as Request.Close
  https://golang.org/cl/24982

Fixes golang/go#16481

Change-Id: Iaddb166387ca2df1cfbbf09a166f8605578bec49
Reviewed-on: https://go-review.googlesource.com/25282


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>