refactor(AVObject): simplify get Created/UpdatedAt (#568)

`this.get('createdAt')` itself evaluates to
`this.createdAt`,
thus `this.createdAt || this.get('createdAt')` is unnecessary.
The same applies to `getUpdatedAt`.

Author: weakish

src/object.js

@@ -228,15 +228,15 @@ module.exports = function(AV) {
        * @return {Date}
        */
       getCreatedAt: function() {
-        return this.createdAt || this.get('createdAt');
+        return this.createdAt;
       },
 
       /**
        * Returns the object's updatedAt attribute.
        * @return {Date}
        */
       getUpdatedAt: function() {
-        return this.updatedAt || this.get('updatedAt');
+        return this.updatedAt;
       },
 
       /**