FIX Allow Director::$rules like //$Action

In 3.0, doing $Action => SomeController would redirect all action requests
to that default controller. In 3.1, you need to do //$Action => SomeController
but it didnt work - those initial slashes broke matching
Merge pull request #1816 from silverstripe-rebelalliance/bug/emptyrules

FIX Allow Director::$rules like //$Action