net/http: fix ServeMux.Handler on trailing-slash redirect

net/http: fix ServeMux.Handler on trailing-slash redirect

When a match involves a trailing-slash redirect,  ServeMux.Handler now
returns the pattern that matched.

Fixes #73688.

Change-Id: I682d9cc9a3628bed8bf21139b98369ffa6c53792
Reviewed-on: https://go-review.googlesource.com/c/go/+/673815
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>