Properly serialise phone number when storing in session

Previously, we just used force_text() to serialise it.
That is the wrong approach, as we allow changing the standard
display to a regional format. But in the session we should always
include the country code. Luckily the easy fix is to just store things
as an international phone number.

src/oscar/apps/checkout/utils.py

@@ -96,9 +96,10 @@ def ship_to_new_address(self, address_fields):
         self._unset('shipping', 'new_address_fields')
         phone_number = address_fields.get('phone_number')
         if phone_number:
+            # Phone number is stored as a PhoneNumber instance. As we store
+            # strings in the session, we need to serialize it.
             address_fields = address_fields.copy()
-            address_fields['phone_number'] = force_text(
-                address_fields['phone_number'])
+            address_fields['phone_number'] = phone_number.as_international
         self._set('shipping', 'new_address_fields', address_fields)
 
     def new_shipping_address_fields(self):