CL 43779/commit 6a6c792e
broke the builds at tip, and that CL doesn't account for
cases where Redirect is directly invoked with a full URL
that itself has a query string.

Updates #17841

Change-Id: Idb0486bae8625e1f9e033ca4cfcd87de95bc835c
Reviewed-on: https://go-review.googlesource.com/44100Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>