Add "X-Content-Type-Options: nosniff" header to server response

[TheRedTrainer]

In order to improve the security for rdap-server responses and avoid a possibility of being attacked with an XSS attack, it is recommended to include the following header in rdap-server response: "X-Content-Type-Options: nosniff " as it is told in the article "Top 5 REST API Security Guidelines" (http://blog.restcase.com/top-5-rest-api-security-guidelines/)

The server should also send an X-Content-Type-Options: nosniff to make sure the browser does not try to detect a different Content-Type than what is actually sent (can lead to XSS).