net/http: make ServeMux preserve query string during redirects

net/http: make ServeMux preserve query string during redirects

Ensure that the implicitly created redirect
for
  "/route"
after
  "/route/"
has been registered doesn't lose the query string information.

Fixes #17841.

Change-Id: Ib7df9242fab8c9368a18fc0da678003d6bec63b8
Reviewed-on: https://go-review.googlesource.com/43779
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>