If an inbound connection is closed, cancel the outbound http request.

This is particularly useful if the outbound request may consume resources
unnecessarily until it is cancelled.

Fixes #8406

Change-Id: I738c4489186ce342f7e21d0ea3f529722c5b443a
Signed-off-by: Peter Waller <p@pwaller.net>
Reviewed-on: https://go-review.googlesource.com/2320Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>