Creates a payload with no none values.

        :param symbols: currency symbols to request specific exchange rates.
        :type symbols: list or tuple
        :return: a payload.
        :rtype: dict

def _create_payload(self, symbols):
        """ Creates a payload with no none values.

        :param symbols: currency symbols to request specific exchange rates.
        :type symbols: list or tuple
        :return: a payload.
        :rtype: dict
        """
        payload = {'access_key': self.access_key}
        if symbols is not None:
            payload['symbols'] = ','.join(symbols)

        return payload