If conn closes but the request cancel select is still blocked
we must close the connection wait channel.

Updates golang/go#24776 (needs bundle into std for fix)

Change-Id: I7e3ffdf2dd9b127727e24fe262b2f4c5d96fc184
Reviewed-on: https://go-review.googlesource.com/108415
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>