fix a tiny thing

payments/backends/stripe.py

@@ -164,7 +164,7 @@ class StripeBackend(BackendBase):
             subscr = Subscription.objects.get(id=sub_internal_id)
             subscr.status = 'active'
             subscr.backend_extid = sub_id
-            subscr.backend_data['subscription_id'] = sub_id
+            subscr.set_data('subscription_id', sub_id)
             subscr.save()
 
         else:
@@ -225,7 +225,8 @@ class StripeBackend(BackendBase):
                 payment.backend_sub_id = invoice['subscription']
             else:
                 payment.backend_sub_id = invoice['subscription']['id']
-        payment.backend_data = {'event_id': event['id'], 'sub_id': payment.backend_sub_id}
+        payment.set_data('event_id', event['id'])
+        payment.set_data('sub_id', payment.backend_sub_id)
         payment.save()
 
         payment.user.vpnuser.add_paid_time(payment.time)