Previously we sometimes set it to $proxy_add_x_forwarded_for and other
times to $remote_addr, but according to
http://wiki.nginx.org/HttpProxyModule#.24proxy_add_x_forwarded_for
$proxy_add_x_forwarded_for handles this for us -- it will be
$remote_addr if there was no X-Forwarded-For header anyway.
(imported from commit 67dc52250e3e7751b1bf375d1a71d0272475435c)
