title: Express session becomes undefined after few seconds
body: <p>I'm using express-session 1.17.1 in a nodejs project.
  I can store the session correctly but after some seconds, session becomes undefined.</p>
  
  <p>here is the server code:</p>
  
  <pre><code>...
   self.app.use(session({
              secret: conf.SERVER.SECRET_WORD,
              resave: false,
              saveUninitialized: false,
              expires: new Date(Date.now() + 36000000)
            }));
  
  server = http.createServer(self.app);
  ...
  </code></pre>
  
  <p>here is the code to store the session when login is done:</p>
  
  <pre><code>...
   if(!err) {
                  user.password="";
                  req.session.user=user._id.toString();
                  req.session.role=user.role;
                  res.status(200).send(user);
                } else {
                  errorHandler.processError(err, res, log);
                }
  ...
  </code></pre>
  
  <p>here is the code to check if session is null:</p>
  
  <pre><code>...
   if(req.session.user==null){
        errorHandler.processError(res.__("GENERAL.SESSION_NULL"), res, null, 401);
      }else{
  // do something.
  ...
  </code></pre>
  
  <p>When i start the server and i make a first request all works fine and session is not null, it has the user stored. But after a few seconds if i perform the same request, session becomes undefined.
  Increasing expires param doesn't modify the behavior.
  Do you know what happens?</p>
  
  <p>Thanks!</p>