Default to 'shipped' when no ship_status exists 

In our NS instance the shipStatus field does not exist on the ItemFulfillment object. To the `status` key in the shipment json is set to `null`.

Defaulting to 'shipped' fixes the issue for us.

The `shipStatus` field is clearly documented [in the SuiteTalk docs](https://system.netsuite.com/help/helpcenter/en_US/srbrowser/Browser2014_1/schema/record/itemfulfillment.html). No idea why it's missing.