efault, this method decodes the argument as utf-8 and returns
        a unicode string, but this may be overridden in subclasses.

        This method is used as a filter for both `get_argument()` and for
        values extracted from the url and passed to `get()`/`post()`/etc.

        The name of the argument is provided if known, but may be None
        (e.g. for unnamed groups in the url regex).
        