pass JSON to HTTP POST Request

I'm trying to make a HTTP POST request to the google QPX Express API [1] using nodejs and request [2].

My code looks as follows:

    // create http request client to consume the QPX API
    var request = require("request")

    // JSON to be passed to the QPX Express API
    var requestData = {
        "request": {
            "slice": [
                {
                    "origin": "ZRH",
                    "destination": "DUS",
                    "date": "2014-12-02"
                }
            ],
            "passengers": {
                "adultCount": 1,
                "infantInLapCount": 0,
                "infantInSeatCount": 0,
                "childCount": 0,
                "seniorCount": 0
            },
            "solutions": 2,
            "refundable": false
        }
    }

    // QPX REST API URL (I censored my api key)
    url = "https://www.googleapis.com/qpxExpress/v1/trips/search?key=myApiKey"

    // fire request
    request({
        url: url,
        json: true,
        multipart: {
            chunked: false,
            data: [
                {
                    'content-type': 'application/json',
                    body: requestData
                }
            ]
        }
    }, function (error, response, body) {
        if (!error && response.statusCode === 200) {
            console.log(body)
        }
        else {

            console.log("error: " + error)
            console.log("response.statusCode: " + response.statusCode)
            console.log("response.statusText: " + response.statusText)
        }
    })

What I'm trying to do is passing the JSON using the multipart argument [3]. But instead of the proper JSON response I got an error (400 undefined).

When I make a request using the same JSON and API Key using CURL instead, it works fine. So there's nothing wrong with my API key or JSON.

What's wrong with my code?

EDIT:

working CURL example:

i) I saved the JSON which I would pass to my request into a file called "request.json":

{
  "request": {
    "slice": [
      {
        "origin": "ZRH",
        "destination": "DUS",
        "date": "2014-12-02"
      }
    ],
    "passengers": {
      "adultCount": 1,
      "infantInLapCount": 0,
      "infantInSeatCount": 0,
      "childCount": 0,
      "seniorCount": 0
    },
    "solutions": 20,
    "refundable": false
  }
}

ii) then, in the terminal I switched to the directory in which the newly created request.json file was located and run (myApiKey stands for my actual API Key obviously):

curl -d @request.json --header "Content-Type: application/json" https://www.googleapis.com/qpxExpress/v1/trips/search?key=myApiKey

[1] https://developers.google.com/qpx-express/ [2] a http request client designed for nodejs: https://www.npmjs.org/package/request [3] here is an example I found https://www.npmjs.org/package/request#multipart-related [4] QPX Express API is returning 400 parse error

I think the following should work:

// fire request
request({
    url: url,
    method: "POST",
    json: requestData
}, ...

In this case, the Content-type: application/json header is automatically added.