templatefilters: try round-trip utf-8 conversion by json filter (issue4933) As JSON string is known to be a unicode, we should try round-trip conversion for localstr type. This patch tests localstr type explicitly because encoding.fromlocal() may raise Abort for undecodable str, which is probably not what we want. Maybe we can refactor json filter to use encoding module more later. Still "{desc|json}" can't round-trip because showdescription() modifies a localstr object.