@staticmethod
def iso88591(value):
    """Doing automatic conversion to iso-8859-1 strings.

        Converts from utf-8 to iso-8859-1 string. That means, all input value
        of Headers class must be UTF-8 stings.
        """
    try:
        if isinstance(value, str):
            return value.encode('utf-8').decode('iso-8859-1')
    except UnicodeError:
        raise AssertionError(
            'Header name/value must be iso-8859-1 encoded (got {0})'.format
            (value))
    raise AssertionError('Header name/value must be of type str (got {0})'.
        format(value))
