default values for optional_multiple parameters do not work

```
$ __policy-rc.d foobar --action start --action stop --deny
Traceback (most recent call last):
  ...
  File "/usr/lib/python3.3/argparse.py", line 1705, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib/python3.3/argparse.py", line 1737, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python3.3/argparse.py", line 1943, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/lib/python3.3/argparse.py", line 1883, in consume_optional
    take_action(action, args, option_string)
  File "/usr/lib/python3.3/argparse.py", line 1811, in take_action
    action(self, namespace, argument_values, option_string)
  File "/usr/lib/python3.3/argparse.py", line 949, in __call__
    items.append(values)
AttributeError: 'str' object has no attribute 'append'
```