Go 1.7 introduces Request.Context and Response.Uncompressed. Use them
both in the http2 Transport when building for Go 1.7.

Tests are in the main repo.

Updates golang/go#15366
Updates golang/go#15134

Change-Id: I4b78a2710b6cf30c769c7f85cea15ecde653a317
Reviewed-on: https://go-review.googlesource.com/23002Reviewed-by: Andrew Gerrand <adg@golang.org>