Respond case insensitively to A and SOA requests (#152)

* When appending the SOA for authoritative NXDOMAIN responses, it needs to go in
the Authoritative section, not the Answer section.

This fixes the acme-dns validation for the lego Let's Encrypt client.

* Respond case-insensitively to A and SOA requests. Add corresponding tests.

This fixes the autocert feature with Let's Encrypt, because Let's Encrypt does
a lookup for the A record with a deliberately mangled case.