Handlers can be registered for specific hosts by specifying the host as
part of the mux pattern. If a trailing slash route is registered for
these host-based patterns, shouldRedirect should indicate that
a redirect is required.

This change modifies shouldRedirect to also take the host of the
request, and now considers host-based patterns while determining if
a request should be redirected.

Fixes #23183
Change-Id: If8753e130d5d877acdc55344833e3b289bbed2b4
Reviewed-on: https://go-review.googlesource.com/84695Reviewed-by: Kunpei Sakai <namusyaka@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>