correct \lithium\net\http\Request::env('REMOTE_ADDR') when $_SERVER['HTTP_X_FORWARDED_FOR'] is specified to return only a single ip address as expected

see http://stackoverflow.com/questions/753645/how-do-i-get-the-correct-ip-from-http-x-forwarded-for-if-it-contains-multiple-ip
and http://en.wikipedia.org/wiki/X-Forwarded-For
Merge pull request #843 from pixelcog/remote-addr

ensure Request::env('REMOTE_ADDR') consistently returns only one IP address