If the client sends a request with a custom Host header and receives
a relative redirect in response, the second request should use the
same Host header as the first request. However, if the response is
an abolute redirect, the Host header should not be preserved. See
further discussion on the issue tracker.

Fixes #22233

Change-Id: I8796e2fbc1c89b3445e651f739d5d0c82e727c14
Reviewed-on: https://go-review.googlesource.com/70792Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>