net/http: make Client copy headers on redirect

net/http: make Client copy headers on redirect

Copy all of the original request's headers on redirect, unless they're
sensitive. Only send sensitive ones to the same origin, or subdomains
thereof.

Fixes #4800

Change-Id: Ie9fa75265c9d5e4c1012c028d31fd1fd74465712
Reviewed-on: https://go-review.googlesource.com/28930
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Francesc Campoy Flores <campoy@golang.org>
Reviewed-by: Ross Light <light@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>