In the situation where the Client.Jar is set and the Request.Header
has cookies manually inserted, the redirect logic needs to be
able to apply changes to cookies from "Set-Cookie" headers to both
the Jar and the manually inserted Header cookies.

Since Header cookies lack information about the original domain
and path, the logic in this CL simply removes cookies from the
initial Header if any subsequent "Set-Cookie" matches. Thus,
in the event of cookie conflicts, the logic preserves the behavior
prior to change made in golang.org/cl/28930.

Fixes #17494
Updates #4800

Change-Id: I645194d9f97ff4d95bd07ca36de1d6cdf2f32429
Reviewed-on: https://go-review.googlesource.com/31435Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>