explicitly set stop_date as None in subscription case.

so that getStopDate() always return the latest state change date.

bt5/erp5_free_subscription/WorkflowTemplateItem/portal_workflow/free_subscription_interaction_workflow/scripts/Request_updateFreeSubscription.xml

@@ -70,6 +70,7 @@ elif request.getFreeSubscriptionRequestType() == "subscription":\n
       effective_date=now)\n
     request.setFollowUpValue(free_subscription)\n
   free_subscription.setStartDate(now)\n
+  free_subscription.setStopDate(None)\n
   free_subscription.validate()\n
 </string> </value>
         </item>