Writing the PUSH_PROMISE can happen concurrently with the request
handler, so we should pass the request handler a cloned Header..

Updates golang/go#18326

Change-Id: I918e7ffbc02459b9ede3226f4bd3c56a6a3b350f
Reviewed-on: https://go-review.googlesource.com/34493
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>