http2: send client trailers
http2: send client trailers

Change-Id: I9cb50eeb3f183f4237d7ba123b8123582fd37882
Reviewed-on: https://go-review.googlesource.com/17912
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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