another approach to fix #40 (return 403 on false Content-Type)

When an attacker uses a <form> to downvote a comment, the browser
*should* add a `Content-Type: ...` header with three possible values:

    * application/x-www-form-urlencoded
    * multipart/form-data
    * text/plain

If the header is not sent or requests `application/json`, the
request is not forged (XHR is restricted by CORS separately).