def set_utf8_without_bom(sub_unicode):
    """Convert a unicode object to plain string.

    Remove BOM header if exists

    Args:
        sub_unicode: any unicode object intended to be converted
                     to string
    Returns:
        sub_str: Plain string encoded in UTF-8 without BOM
    """
    if sub_unicode.startswith(u'\ufeff'):
        sub_unicode = sub_unicode[3:]
    sub_str = sub_unicode.encode('utf-8')
    return sub_str
