The http2 Transport now respects the http1 Transport's
DisableCompression, DisableKeepAlives, and ResponseHeaderTimeout, if
the http2 and http1 Transports are wired up together, as they are in
the upcoming Go 1.6.

Updates golang/go#14008

Change-Id: I2f477f6fe5dbef9d0e5439dfc7f3ec2c0da7f296
Reviewed-on: https://go-review.googlesource.com/18721Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>