xhrInterceptor support async response

[yucopowo]

XMLHttpRequestInterceptor

if (response instanceof Promise) {
    this.readyState = readyState;
    this.status = status;
    //TODO: should 'this.response' be the response string or the response json?
    response.then(function(data) {
        payload = data;
        if (data instanceof _Response.Response) {
            var responseHeaders = data.headers;

            payload = data.body;
            status = data.code;

            Object.assign(_this._responseHeaders, responseHeaders);
        }

        var responseString = JSON.stringify(payload);
        _this.responseText = _this.response = responseString;

        var requestDelay = helpers.getDelay();
        if (!requestDelay) {
            // for backwards compatibility if there is no requestDelay ot it 0 return callback without setTimeout
            successCallback.call(_this);
        } else {
            return setTimeout(function() {
                successCallback.call(_this);
            },
            requestDelay);
        }
    });

    return setTimeout(function() {},
    0);
}

[yucopowo]

router.get('/async', function (request) {
    return new Promise(function (resolve, reject) {
        resolve({
            body:{
                a:1
            }
        });
    });