http2: add Transport.RoundTripOpt, adds option to RoundTrip without new dials
http2: add Transport.RoundTripOpt, adds option to RoundTrip without new dials

For integration with the net/http.Transport.

Updates golang/go#6891

Change-Id: I7a44e4c4259aa57cec1b54666c333b5fc519caf8
Reviewed-on: https://go-review.googlesource.com/16692
Reviewed-by: Blake Mizerany <9064db5bb6ad06c7c3afee8dc1334281c314ed63@gmail.com>

cesanta/goxnet,cesanta/goxnet,cmars/tools,cmars/tools,DramaFever/golang.org-x-net,DramaFever/golang.org-x-net,albertjin/golang-net,cmars/tools,albertjin/golang-net,golang/net,cmars/tools