http2: export Server.ServeConn

Fixes golang/go#12737
Updates golang/go#14141

Change-Id: I552b603b63a7c87d7fcdb4eb09f96ab9fd0ec0aa
Reviewed-on: https://go-review.googlesource.com/19176
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
4 files changed