Add ecomm/order-add v2.0 method

[kstefan]

http://apidocs.mailchimp.com/api/2.0/ecomm/order-add.php

[basiliscus]

The same issue with this request:

1. All fields of type double should be converted to Double.
2. All fields of type int should be converted to Integer.
3. The field order_date should be of type Date instead of String.

[kstefan]

I fixed field types and retested the method.