Changed call of isGranted

This didn't work for me as it was.
The function header for isGranted is as follows:

    final public function isGranted($attributes, $object = null)

It was called with $attributes = array(Expression, ModelClass) and $object was always = null

This didn't work, as the object was always null and always returned true.
Merge pull request #319 from kufi/patch-1

Changed call of isGranted