Add a method to expose the handler to allow it to be installed at a
non-standard location or used with a different ServeMux.

fixes #15030

Change-Id: If778ad6fcc200f124a05c0a493511e364fca6078
Reviewed-on: https://go-review.googlesource.com/24722Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>