Add params, query and fragment when redirecting to a language or default route

[flaviocopes]

Fix an issue in the Grav lang redirect. E.g. I call /order/id:order-20160320-202557-668641/token:2mgb4D27S3, if multilang is ON, and the lang is prefixed, Grav loads the page /en/order/, losing all the parameters and additional URL segments

[rhukster]

One thing we could do to optimize. Rather than check if $url == '/' just ltrim($url, '/')