Using 'choices' in add_argument leads to invalid swagger schema

[leopold-p]

I seems the issue is inherited from flask-restplus
noirbizarre/flask-restplus#488

Code

In Argument from regparse.py:

    @property
    def __schema__(self):
        if self.location == 'cookie':
            return
        param = {
            'name': self.name,
            'in': LOCATIONS.get(self.location, 'query')
        }
        _handle_arg_type(self, param)
        if self.required:
            param['required'] = True
        if self.help:
            param['description'] = self.help
        if self.default is not None:
            param['default'] = self.default() if callable(self.default) else self.default
        if self.action == 'append':
            param['items'] = {'type': param['type']}
            param['type'] = 'array'
            param['collectionFormat'] = 'multi'
        if self.action == 'split':
            param['items'] = {'type': param['type']}
            param['type'] = 'array'
            param['collectionFormat'] = 'csv'
        if self.choices:
            param['enum'] = self.choices
            param['collectionFormat'] = 'multi'
        return param

An argument with choices must not generate a collectionFormat field