Fix Issue #496

[dongweiming]

I have same problem like #496 in Python 3.

The argparse.py#L1430 module said:

if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
    kwargs['required'] = True

[coveralls]

Coverage increased (+0.04%) to 93.595% when pulling c53a778 on dongweiming:fix-496 into 5efc901 on jkbrzt:master.

[coveralls]

Coverage increased (+0.04%) to 93.595% when pulling c53a778 on dongweiming:fix-496 into 5efc901 on jkbrzt:master.

[jkbrzt]

Thanks, @dongweiming!