Jasmine AJAX mock turns string into array

I'm trying to write a test suite around my AjaxRequest class, but when I'm trying to inspect the request body I get this test failure

FAILED TESTS:
AjaxRequest
  #POST
  ✖ attaches the body to the response
    PhantomJS 1.9.8 (Mac OS X 0.0.0)
  Expected Object({ example: [ 'text' ] }) to equal Object({ example: 'text' }).

Here's the relevant bit of the unit test:

      req = new AjaxRequest().post('http://example.com')
            .body({
                example: 'text'
            }).run();

And here's the run() method where the ajax request is made

var options = {
        url: this._url,
        method: this._method,
        type: 'json',
        data: this._body
    };

    return when(reqwest(options));

I'm using reqwest to issue ajax requests.

Could someone point out why it's expecting ['text'] when the request sent 'text' in the json body?

Thank you!

Changing the implementation of AjaxRequest solved the problem.

Here is the new implementation of run using XMLHttpRequest

run () {
    var req = new XMLHttpRequest();

    req.open(this._method, this._url, true);

    req.send(JSON.stringify(this._body));

    return when.promise((resolve, reject) => {
        req.onload = function() {
            if (req.status < 400) {
                var param = req.response;
                try { param = JSON.parse(param) } catch (e) { };
                resolve(param);
            } else {
                reject(new RequestError(req.statusText, req.status));
            }
        };
    });
}

This not only gets rid of an extra library but also leaves more control over when to reject a request promise.