net/http: make Client propagate Request.Cancel over redirected requests

net/http: make Client propagate Request.Cancel over redirected requests

On HTTP redirect, the HTTP client creates a new request and don't copy
over the Cancel channel. This prevents any redirected request from being
cancelled.

Fixes #14053

Change-Id: I467cdd4aadcae8351b6e9733fc582b7985b8b9d3
Reviewed-on: https://go-review.googlesource.com/18810
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>