Clear record timestamp after save

Since changeset 28a09c279d67, the timestamp is sent also on button calls but if
the record is saved just before this generate an concurrency exception. Indeed
the _timestamp must be cleared just after the save (because the record is not
reloaded before the button call).

issue4644
review8021002

tryton/gui/window/view_form/model/record.py

@@ -270,6 +270,7 @@ class Record(SignalEvent):
     def cancel(self):
         self._loaded.clear()
         self.modified_fields.clear()
+        self._timestamp = None
 
     def get_timestamp(self):
         result = {self.model_name + ',' + str(self.id): self._timestamp}
@@ -311,8 +312,7 @@ class Record(SignalEvent):
                             [self.id], value, context=context)
                     except RPCException:
                         return False
-            self._loaded.clear()
-            self.modified_fields = {}
+            self.cancel()
             if force_reload:
                 self.reload()
             if self.group: