ipv6 addresses need to be quoted in YAML output

@edmonds pointed out in #3501 (closed):

Side note, I recently learned that YAML doesn't like colons in unquoted strings, so some YAML parsers reject things like:

query_address: ::

I fixed that by quoting IP addresses in dnstap-ldns (https://github.com/dnstap/dnstap-ldns/pull/9), looks like BIND's dnstap-read has the same issue.

He's correct, and dig and mdig also have the same issue. (Possibly delv too, but I don't think so.)