The path of the new stripped URL should also be cleaned. Since an empty path
may cause unexpected errors in some HTTP handlers, e.g. http.ServeFile.

Fixes #30165

Change-Id: Ib44fdce6388b5d62ffbcab5266925ef8f13f26e2
Reviewed-on: https://go-review.googlesource.com/c/161738Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>