Backorder shipping in admin system not working

For a store that doesn't track inventory at all, we set the following in our initialization:

```
config.track_inventory_levels = false
config.allow_backorder_shipping = true
```

Now, in this scenario:

1. Make an order through the regular end-user checkout process

2. In admin, add an item to the order.  This item shows up as backordered.

3. Add a payment so the revised order is paid, and should be ready to ship

4. Can't ship the order - there's no "ship" button.

This PR edits the way the shipment state is calculated, by not leaving it in "pending" state because of backordered inventory units if we allow backordered orders to ship.

That said - I'm sure there's a better way to do this.  Ideally, adding an item in the admin system would always show it as "on-hand" if we're not tracking inventory.  After a little checking, though, it wasn't obvious to me how to accomplish that...