NodeJS Express session variable not saved when using jQuery POST

OK, so this questions has been asked a bunch of times and in different manners, however, none of the proposed solutions have worked for me so far. I have a very simple log-in page that passes user name and password to my server and the server authenticates the user and saves the user object in the session:

App.js:

var express = require('express')
              , SessionMongooseStore = require("session-mongoose")(express)
              , SessionStore = new SessionMongooseStore({
                               url: "mongodb://localhost/MySessions",
                               interval: 1200000 
                });

var app = express();
app.set('views', __dirname + '/views');
app.set('view engine', 'ejs');
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.bodyParser());
app.use(express.methodOverride());
app.use(express.cookieParser());

app.use(express.session({
        store: SessionStore,
        cookie: {maxAge: 36000000},
        secret: 'MySecret'
    }));

app.use(function(req, res, next) {
   res.header('Access-Control-Allow-Credentials', true);
   res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
   res.header('Access-Control-Allow-Headers', 'X-Requested-With, X-HTTP-Method-  Override, Content-Type, Accept');
   next();
});

Route:

app.post('/login',Session.LogIn);

Session.LogIn definition:

exports.LogIn = function(req,res){
    if(req.body){
       var user_name = req.body.email_address;
       var pwd = req.body.password;
       Store.getUser(user_name,pwd,function(err,user){
          if(error) { //do something } 
          else{
            //SAVING DATA TO THE SESSION
            req.session.authenticatedUser = user;
            req.session.foo = 'bar';
            req.session.save();
            var successResponse = {};
            successResponse.redirect = '/redirect';
            res.json(successResponse);
          }
       });
    }
}

My jQuery AJAX POST:

var post = $.ajax({url: '/login', 
                 type: "POST", 
                 data: querystring, 
                 xhrFields: {withCredentials: true}, 
                 success: function(data) {
                    if(data.code!=200){
                        alert('Log in failed');
                    }
                    else if(data.redirect){
                        window.location = data.redirect;
                    }
                    else{
                        alert('Could not understand response from the server');
                    }
            }});

When I make the POST, I can see the session variables 'user' and 'foo' being set, I can print them, but when I come back to any route, the variables are undefined. Example, calling '/' after logging in:

 app.get('/',function(req,res){
    console.log('--------- Looking for authenticated user saved in session: ' +   req.session.authenticatedUser);
    console.log('--------- Foo: ' + req.session.foo);
    res.render('index');
 });

I get:

--------- Looking for authenticated user saved in session: undefined
--------- Foo: undefined

I have attempted to reproduce this using HTTP Poster add-on for FF and doing a POST, not using jQuery, and it works fine, the variables are saved in the session object. Every POST I've read talks about using

xhrFields: {withCredentials: true}

with my jQuery POST and using:

 res.header('Access-Control-Allow-Credentials', true);

in my node js server, which I am doing as you can see from the code above, but still no luck. Any ideas?