Modified sale line serialization for cart #7211

By default, the standard fields 'display_name', 'url', 'image' are
picked from the sale line's product. In downstream modules this
behaviour can be modified.

sale.py

@@ -130,8 +130,6 @@ def _add_or_update(self, product_id, quantity, action='set'):
         values.update(SaleLine(**values).on_change_quantity())
 
         for key, value in values.iteritems():
-            # This can be removed in 3.4 and call on_change on
-            # instance
             if '.' not in key:
                 setattr(order_line, key, value)
         return order_line
@@ -164,6 +162,13 @@ def serialize(self, purpose=None):
         if purpose == 'cart':
             res.update({
                 'id': self.id,
+                'display_name': self.description or self.product.name,
+                'url': self.product.get_absolute_url(_external=True),
+                'image': (
+                    self.product.default_image.transform_command().thumbnail(
+                        150, 150, 'a'
+                    ).url() if self.product.default_image else None
+                ),
                 'product': self.product.serialize(purpose),
                 'quantity': number_format(self.quantity),
                 'unit': self.unit.symbol,