Dropping default value for __call

The `__call` method's `$args` parameter is required and will always be present as an array (http://www.php.net/manual/en/language.oop5.overloading.php#object.call), adding a default value (like `$args = array()`) is not needed and even causes problems, as described in padraic/mockery#263.

Removing this allows us to manipulate the object as expected.