cope with the retired nerd response codes

I do not think that tw=158 is really retired, but Locale::Codes::Country
does, so there you go.
sort out the uk/gb problem more thoroughly
v0.010

        - provide a nerd_dk response even for retired Taiwan; some other
          countries are not yet handled; for more information, see
          https://rt.cpan.org/Ticket/Display.html?id=92303
        - more clearly handle the gb-vs-uk conflict by making "code" always
          mean ISO code