Posting JSON data to a webserver gives error 415 (Unsupported media type 'x-www-form-urlencoded' in request.)

[redM0nk]

I'm using this library to make a POST request to ubidots's server via ESP8266 (ESP-LINK).
One of the requirements is that it only accepts JSON data.
I've tried the following method:

    rest.setHeader("\"Content-Type\":\"application/json\"");
    rest.setContentType("json");
    rest.request(path_data, "POST", "{\"data\":8}");

Output:

ubidots: POST failed with error 415
Response: {"detail": "Unsupported media type 'x-www-form-urlencoded' in request."}

Any help is appreciated.