As of https://golang.org/cl/21530, rules are updated to state
that Handlers shouldn't modify the provided Request. This change
updates StripPrefix to follow that rule.

Resolves #18952.

Change-Id: I29bbb580722e871131fa75a97e6e038ec64fdfcd
Reviewed-on: https://go-review.googlesource.com/36483Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>