Missing response value on ajax result.

Hi guys,

I'm trying to do a simple ajax call and get a response but i get an answer with null response value.

RxJS version:
"rxjs": "^5.0.0-beta.12"

Code to reproduce:

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/dom/ajax';

 Observable.ajax({
        url: window.location.origin + "/api/book/load",
        method: "POST",
        responseType: "json",
        body: {
            key: "userKey",
            sessionId: "oisudofiu"
        },
        headers: {
            "Accept": "application/json, text/plain, */*",
            "cache-control": "no-cache"
        }
    }).subscribe(
        xhr => console.log("LOADED ", JSON.stringify(xhr, null, 2)),
        error => console.log("ERROR ", JSON.stringify(error, null, 2)),
        () => console.log("COMPLETE"))

Expected behavior:
Get a response with result information

Actual behavior:
Missing response information on Ajax response

{
  "originalEvent": {
    "isTrusted": true
  },
  "xhr": {},
  "request": {
    "async": true,
    "crossDomain": false,
    "withCredentials": false,
    "headers": {
      "Accept": "application/json, text/plain, */*",
      "cache-control": "no-cache",
      "X-Requested-With": "XMLHttpRequest",
      "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8"
    },
    "method": "POST",
    "responseType": "json",
    "timeout": 0,
    "url": "http://localhost:8888/api/book/load",
    "body": "key=userKey&sessionId=oisudofiu"
  },
  "status": 200,
  "responseType": "json",
  "response": null
}

Additional information:
If i make the same call with axios against the same (jersey) REST endpoint I get back the correct data. The endpoint is configured to ignore params and respond with "Result of book loaded" string. Here is the axios response:

{
  "data": "\"Result of book loaded\"",
  "status": 200,
  "statusText": "OK",
  "headers": {
    "date": "Wed, 05 Oct 2016 16:27:40 GMT",
    "cache-control": "no-cache",
    "expires": "Mon, 01 Jan 1990 00:00:00 GMT",
    "server": "Development/1.0",
    "content-length": "27",
    "content-type": "application/json;charset=utf-8"
  },
  "config": {
    "transformRequest": {},
    "headers": {
      "Accept": "application/json, text/plain, */*",
      "Content-Type": "application/x-www-form-urlencoded"
    },
    "timeout": 0,
    "xsrfCookieName": "XSRF-TOKEN",
    "xsrfHeaderName": "X-XSRF-TOKEN",
    "maxContentLength": -1,
    "method": "post",
    "baseURL": "http://localhost:8888/api",
    "url": "http://localhost:8888/api/book/load",
    "data": "key=user&sessionId=password"
  },
  "request": {}
}

Thanks a lot for any help.

(EDIT by @blesh to add code color formatting)

Start with the Observable.ajax entry point and reproduce the POST request shown in the issue using the stated RxJS version. Compare the Ajax response with the endpoint's JSON result and verify that the response field contains the returned value rather than null.