Request.Cancel was added in Go 1.5.

Change-Id: Iff28824315a58956fcfcb36d9bca6228cf6ea4f6
Reviewed-on: https://go-review.googlesource.com/17822Reviewed-by: Qi Zhao <zhaoq@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>