This commit replaces a call to call_user_func_array() in WC_Data_Store::__call() with argument unpacking using the spread operator which was introduced in PHP 5.6. This change should improve WooCommerce performance a tiny bit since WC_Data_Store::__call() is called somewhat frequently and call_user_func_array() has a bad performance reputation. I added one unit test to make sure this change doesn't break the functionality of the altered method.