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.