Missing error handling for Content-type header

[Hugo1380]

For POST /dns/:hostname, if you haven't set `Content-Type: application/json' in the request.
You will get an error telling you that a list of IP addresses is required.
It may be more relevant to check if the Content-Type header is present/valid and return a missing header response or unsupported_media_type (code 415).

Example:

curl -X POST -d '{"ips": ["192.168.0.2"]}' 127.0.0.1:8080/dns/test2
{"error":"IP address list is required"}