Add missing _order_name on invoice

invoice.py

@@ -237,6 +237,7 @@ class Invoice(OSV):
     'Invoice'
     _name = 'account.invoice'
     _description = __doc__
+    _order_name = 'number'
     company = fields.Many2One('company.company', 'Company', required=True,
             states=_STATES)
     type = fields.Selection(_TYPE, 'Type', select=1, on_change=['type'],