This is required to work with the upcoming rewrite of the
httptest.Server's connection tracking in
https://go-review.googlesource.com/#/c/15151/
which (at least as of patchset 7) doesn't forcefully tear
down a StateNew connection during Server.Wait. That might
change.

In any case, this adds support for ConnState, which users would
expect regardless of protocol in a mixed HTTP/1 and HTTP/2
environment.

Change-Id: I124aafec29dda123a018935fa306f465ae99cd97
Reviewed-on: https://go-review.googlesource.com/15913Reviewed-by: Andrew Gerrand <adg@golang.org>