The existing transport implementation limit scheme to https, so
simple h2c can not be used, if we loose the limit to http scheme,
it works well with existing h2c server support.

Fixes golang/go#15830

Change-Id: Icfe65d72b3e6fb41f69019539fb5b86144814378
Reviewed-on: https://go-review.googlesource.com/23181Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>