def add_header(name, value):
    """Similar to set_header() except that, if there is already a response
        header by ``name`` present, ``value`` will be appended to existing
        value using ',' seperator. Returns the new value for header name as
        byte-string.

        ``name``,
            byte-string of header field name, in lower case.

        ``value``,
            Any type which can be converted to string.
        """
