Allow additional header params,

    :param str software_version: the version of the software

    """

    def __init__(self, timestamp=None, version=SPEC_VERSION, software='sbws',

                 software_version=__version__):

                 software_version=__version__, **kwargs):

        self.timestamp = timestamp or int(time.time())

        self.version = version

        self.software = software

        self.software_version = software_version

        if kwargs.get('earlier_result_ts'):

            self.earlier_result = kwargs['earlier_result_ts']

    def __str__(self):

        """Return header string following spec version 1.1.0."""

        frmt = '{timestamp}\nversion={version}\nsoftware={software}\n' \

               'software_version={software_version}\n'

        return frmt.format(**self.__dict__)

        # sorting the list to generate determinist headers

        kv_headers = ' '.join(sorted(['='.join([k, str(v)])

                                      for k, v in self.__dict__.items()]))

        header = '\n'.join([str(self.timestamp), kv_headers, ''])

        log.debug('header %s', header)

        return header