Despite the previously known behavior of Request.WithContext
shallow copying a request, usage of the request inside server.ServeHTTP
mutates the request's URL. This CL implements deep copying of the URL.

Fixes #20068

Change-Id: I86857d7259e23ac624d196401bf12dde401c42af
Reviewed-on: https://go-review.googlesource.com/41308
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>