[FIX] calendar: prevent updating event with a start date greater than…

… the end date

Setting a constraint on an old api style computed field is broken in Odoo 8.0
The constraint is checked with the old value of the computed field, not the new one
So, it was possible to update an event with a start date greater than the stop date
And once done, it was impossible to correct the mistake

addons/calendar/calendar.py

@@ -971,12 +971,14 @@ def _get_default_partners(self, cr, uid, ctx=None):
 
     def _check_closing_date(self, cr, uid, ids, context=None):
         for event in self.browse(cr, uid, ids, context=context):
-            if event.stop < event.start:
+            if event.start_datetime and event.stop_datetime < event.start_datetime:
+                return False
+            if event.start_date and event.stop_date < event.start_date:
                 return False
         return True
 
     _constraints = [
-        (_check_closing_date, 'Error ! End date cannot be set before start date.', ['start', 'stop'])
+        (_check_closing_date, 'Error ! End date cannot be set before start date.', ['start_datetime', 'stop_datetime', 'start_date', 'stop_date'])
     ]
 
     def onchange_allday(self, cr, uid, ids, start=False, end=False, starttime=False, endtime=False, startdatetime=False, enddatetime=False, checkallday=False, context=None):