RRSIG support

[moparisthebest]

Running dig @127.0.0.1 -p 8853 rrsig debian.org pointed to dingo gives this log:

$ dingo-linux-amd64 -port 8853
2017/09/24 03:38:07 starting 10 Google Public DNS client(s) querying server 216.58.195.78
2017/09/24 03:38:07 dingo ver. 0.13 listening on 127.0.0.1 UDP port 8853
2017/09/24 03:38:11 resolving debian.org./RRSIG
2017/09/24 03:38:11 Pack() failed: dns: nil rr

And does not return any data, check what should be returned by running dig @8.8.8.8 rrsig debian.org

secureoperator has the same bug here

https://github.com/chenhw2/google-https-dns seems to work for RRSIG