Ensure get_discount uses the correct parent charge for discount

Before this, it was wrongly using the tax inclusive charge each time.

oscar/apps/shipping/methods.py

@@ -47,7 +47,7 @@ def is_discounted(self):
         # We check to see if the discount is non-zero.  It is possible to have
         # zero shipping already in which case this the offer does not lead to
         # any further discount.
-        return self.get_discount()['discount'] > 0
+        return self.discount > 0
 
     @property
     def discount(self):
@@ -68,15 +68,14 @@ def description(self):
     def get_discount(self):
         # Return a 'discount' dictionary in the same form as that used by the
         # OfferApplications class
-        parent_charge = self.method.charge_incl_tax
         return {
             'offer': self.offer,
             'result': None,
             'name': self.offer.name,
             'description': '',
             'voucher': self.offer.get_voucher(),
             'freq': 1,
-            'discount': self.offer.shipping_discount(parent_charge)}
+            'discount': self.effective_discount}
 
     @property
     def charge_incl_tax_before_discount(self):
@@ -90,6 +89,13 @@ def charge_excl_tax_before_discount(self):
     def is_tax_known(self):
         return self.method.is_tax_known
 
+    @property
+    def effective_discount(self):
+        """
+        The discount value.
+        """
+        raise NotImplemented()
+
     @property
     def charge_excl_tax(self):
         raise NotImplemented()
@@ -101,6 +107,11 @@ def charge_incl_tax(self):
 
 class TaxExclusiveOfferDiscount(OfferDiscount):
 
+    @property
+    def effective_discount(self):
+        parent_charge = self.method.charge_excl_tax
+        return self.offer.shipping_discount(parent_charge)
+
     @property
     def charge_excl_tax(self):
         parent_charge = self.method.charge_excl_tax
@@ -117,6 +128,11 @@ def charge_incl_tax(self):
 
 class TaxInclusiveOfferDiscount(OfferDiscount):
 
+    @property
+    def effective_discount(self):
+        parent_charge = self.method.charge_incl_tax
+        return self.offer.shipping_discount(parent_charge)
+
     @property
     def charge_incl_tax(self):
         parent_charge = self.method.charge_incl_tax