Format a json string to csv like.
    :param json_str: json object string
    :param show_header: can returns csv header line
    :param separator: csv column format separator
    :return: if show_header=False: a string like csv formatting;
             if show_header=True: a tuple (header, csv string)

def json2csv(json_str, show_header=False, separator='\t'):
    """
    Format a json string to csv like.
    :param json_str: json object string
    :param show_header: can returns csv header line
    :param separator: csv column format separator
    :return: if show_header=False: a string like csv formatting;
             if show_header=True: a tuple (header, csv string)
    """
    json_obj = json.loads(json_str)
    cols = [col for col in json_obj.keys()]
    vals = [str(json_obj.get(col)) for col in cols]
    header = None
    if show_header:
        header = separator.join(cols)
    values = separator.join(vals)

    return (header, values) if show_header else values