[SalesForce] HTTP POST Response Body is blank – 204 status

I've changed the endpoint url and the json payload for security reasons but let's assume that I've tested the endpoint using advanced rest client and with the same json in my apex code and it works.

myJson: {
  "user" : "test@force.com",
  "pass" : "mypass",
  "url" : "https://login.salesforce.com/services/Soap/u/37.0",
  "type" : "New"
}

My callout looks like this

    HttpRequest req = new HttpRequest();
    HttpResponse res = new HttpResponse();
    Http http = new Http();

    String SessionId = Userinfo.getSessionId();
    String strURL = System.URL.getSalesforceBaseURL().toExternalForm();
    req.setEndpoint('https://myapp.herokuapp.com/webapi/myapi');
    req.setMethod('POST');
    req.setHeader('Content-Type', 'application/json');
    JSONGenerator gen = JSON.createGenerator(true);
        gen.writeStartObject();
        gen.writeStringField('user', user);
        gen.writeStringField('pass', pass);
        gen.writeStringField('url', url);
        gen.writeStringField('Type',Type);
        gen.writeEndObject();              
        String myJson = gen.getAsString();
        req.setBody(myJson);

    try {
        req.setTimeout(120000);
        res = http.send(req);
    }

Here are my request and response

CALLOUT_REQUEST|System.HttpRequest[Endpoint=https://myapp.herokuapp.com/webapi/myapi, Method=POST]
CALLOUT_RESPONSE|System.HttpResponse[Status=No Content, StatusCode=204]

I'm having trouble identifying why this isn't working. The issue is, the response is empty. it's supposed to return a json.