Get default headers for API request
        :param unicode content_type: HTTP request content type
        :param: str token: customer app ID
        :return: dictionary

def _default_headers(self, content_type=CONTENT_TYPE, token=None):
        """
        Get default headers for API request
        :param unicode content_type: HTTP request content type
        :param: str token: customer app ID
        :return: dictionary
        """
        headers = {
            self.HEADER_CLIENT_TYPE: self.CLIENT_TYPE,
            self.HEADER_TIMESTAMP: str(self._now()),
            self.HEADER_CUSTOMER_APP_ID: token or self._token
        }

        if content_type is not None or content_type is not False:
            headers[self.HEADER_CONTENT_TYPE] = content_type

        return headers