Exclude ItemFulfillment items that are not associated with a line item 

When fulfilling a KitItem in NetSuite it is broken out into its components. So if an item is purchased in Spree which is a KitItem in NetSuite composed of two components the shipment payload will contain 3 items (the original kit item plus its two components).

This fixes the issue by filtering out all items in the ItemFullfilment object which are not associated with a SO line item.