We used to do it sometimes as an optimization, but the optimization is
flawed: in all non-contrived cases we need to deep clone the map
anyway. So do it always, which both simplifies the code but also fixes
the X-Forward-For value leaking to the caller's Request, as well as
modifications from the optional Director func.

Fixes #18327

Change-Id: I0c86d10c557254bf99fdd988227dcb15f968770b
Reviewed-on: https://go-review.googlesource.com/46716Reviewed-by: Ian Lance Taylor <iant@golang.org>