Broken for certain paths

On Spree 1.3.1, if you try to override a taxon page like

/t/categories/dogs

by the time the pages are queried for the slug, the slug will (incorrectly) be

/tcategories/dogs

This is due to the nature of the regex being used to remove the spree mount point. Fixed this by modifying the regex to only match the beginning of the string.

This should be merged into master and 1-3-stable...