Fix support for CakePHP not on base URL ('/')

Commit a25ed59890bedc427b929c91025496305eaef948 broke support for not-on-base-URL CakePHP deployments. For example, if my cake installation resides at `/cake`, `Helper::assetUrl` generates URLs in the form `/cake/css/...`, which are note recognized by the Minify library.

This fixes it by adjusting the file paths if `$this->request->base` is non-empty.