Fix bug when editing a data row

This was a new-ish bug, caused I think by 4a2ba3925d

tailbone/views/master.py

@@ -1205,6 +1205,7 @@ class MasterView(View):
             else:
                 kwargs['cancel_url'] = self.get_row_action_url('view', instance)
 
+        kwargs.setdefault('session', self.Session())
         form = forms.AlchemyForm(self.request, fieldset, **kwargs)
         form.readonly = self.viewing
         return form