[FIX] stock: picking action_cancel multi

As the function 'action_cancel' is multi, we cannot  set the value
'is_locked' to True by using '='. Because it will raise an error if self
contains more or less than 1 record.

To fix this we call the 'write' method which support all cases

addons/stock/models/stock_picking.py

@@ -543,7 +543,7 @@ class Picking(models.Model):
     @api.multi
     def action_cancel(self):
         self.mapped('move_lines')._action_cancel()
-        self.is_locked = True
+        self.write({'is_locked': True})
         return True
 
     @api.multi