Fix of absent discount attribute in shipping models

It's necessary for place_order in oscar.apps.order.utils [here](https://github.com/tangentlabs/django-oscar/blob/master/oscar/apps/order/utils.py#L82):
```python
…
            if application['result'].affects_shipping:
                # Skip zero shipping discounts
                if shipping_method.discount <= D('0.00'):
                    continue
                # If a shipping offer, we need to grab the actual discount off
                # the shipping method instance, which should be wrapped in an
                # OfferDiscount instance.
                application['discount'] = shipping_method.discount
…
```
Purposes of existence of this attribute (or property) is not clear for me. Where it must be used considering that this property is not model field? I saw source code logic depends on it, and found almost nothing. It's always zero, except use case in OfferDiscount model (and inherited TaxExclusiveOfferDiscount, TaxInclusiveOfferDiscount).