Keep ship_method_ref as a simple string

Even though quickbooks api tells it is a ReferenceType I don't any
ShipMethod type on their docs or UI. Setting a name to the shipping
method works just fine though. The name is properly displayed on the
sales receipt page.

Does that make sense?

Sales order reference doc: https://developer.intuit.com/docs/0025_quickbooksapi/0050_data_services/030_entity_services_reference/salesreceipt