fix(routing): URL-decodes path segments like Elgg 1.8

Non-ASCII characters get URL encoded in `$_SERVER['PATH_INFO']` so instead we
copy the URL path into a query string var and use that var to build path
segments in the router. This should yield identical behavior as Elgg 1.8.

Fixes #6218 and adds better router tests.

 * [x] decide between "routing" and "router"
 * [x] change last commit from "fix" to "test"