dns.cgi: Add use Encode + encode back to UTF-8

dns.cgi: Add use Encode + encode back to UTF-8

- use Encode was missed out in the previous patch for dns.cgi This would mean that the
   decode from UTF-8 would fail. I had tested the previous change but forgot to copy across
   the use Encode line when I created the patch.
- This patch adds an encode back to UTF-8 after running the cleanhtml command. This way
   the text is decoded from UTF-8 so that the cleanhtml command works correctly on
   umlauted characters and then is encoded back to UTF-8 so that all text in the cgi page
   is UTF-8.

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>