calculate() takes an optional array as a parameter.

When I pass in the array, I get a warning in my IDE since it's not defined in this class. So I updated the definition.

Author: Tony DeStefano

lib/Refund.php

@@ -14,7 +14,7 @@
  * --------------------------------------------------------------------------
  * Refund -> Custom actions
  * --------------------------------------------------------------------------
- * @method array calculate()      Calculate a Refund.
+ * @method array calculate(array $calculation = null)      Calculate a Refund.
  *
  */
 class Refund extends ShopifyResource
@@ -30,4 +30,4 @@ class Refund extends ShopifyResource
     protected $customPostActions = array (
         'calculate',
     );
-}
+}