net/http: enable HTTP/2 support in DefaultTransport

The GODEBUG option remains, for now, but only for turning it off.
We'll decide what to do with it before release.

This CL includes the dependent http2 change (https://golang.org/cl/16692)
in the http2 bundle (h2_bundle.go).

Updates golang/go#6891

Change-Id: If9723ef627c7ba4f7343dc8cb89ca88ef0fbcb10
Reviewed-on: https://go-review.googlesource.com/16693


Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>