net/http: use relative path in Location redirect

If the cleaned path did not match the requested path, ServeMux.Handler
would return a Location header which reflected the hostname in the
request, possibly leading to an incorrect redirect. Instead the
Location header should be relative, like the other cases in
ServeMux.Handler.

Change-Id: I2c220d925e708061bc128f0bdc96cca7a32764d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/313950


Trust: Roland Shoemaker <roland@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>