Return a message header fragment dict.

        Args:
            request_id (str or None) :
                Message ID of the message this message replies to

        Returns:
            dict : a message header

def create_header(cls, request_id=None):
        ''' Return a message header fragment dict.

        Args:
            request_id (str or None) :
                Message ID of the message this message replies to

        Returns:
            dict : a message header

        '''
        header = {
            'msgid'   : bkserial.make_id(),
            'msgtype' : cls.msgtype
        }
        if request_id is not None:
            header['reqid'] = request_id
        return header