add is_valid fake method to enable beanstream credit cards

Author: Chris7

billing/gateways/beanstream_gateway.py

@@ -95,6 +95,11 @@ def convert_cc(self, credit_card, validate=True):
             credit_card.number,
             credit_card.month, credit_card.year,
             credit_card.verification_value)
+        if not hasattr(card, 'is_valid'):
+            # there is no is_valid on the beanstream credit card object
+            def is_valid():
+                return True
+            card.is_valid = is_valid
         if validate:
             self.validate_card(card)
         return card