If rootdir is '/' or '' it is not added to shortRoute

Useful if you organize your assets by functionality, not by file type, i.e. all common files are in /common dir, all homepage files are in /assets/homepage dir, all 'contacts' page files are in /assets/contacts dir etc. Currently if you set js.root to '/' or '' it will produce '//common/...' routes, this patch fixes it.