This makes x/net/http2's ResponseWriter implement the new interface,
http.Pusher. This new interface requires Go 1.8. When compiled against
older versions of Go, the ResponseWriter does not have a Push method.

Fixes golang/go#13443

Change-Id: I8486ffe4bb5562a94270ace21e90e8c9a4653da0
Reviewed-on: https://go-review.googlesource.com/29439Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>