public ActionResult Login()
        {
            if (Session["KlientId"] != null)
                return RedirectToAction("LoggedIn", "Klient");
            return View();
        }

[HttpPost]
        public ActionResult Login(Klient klient)
        {
            var user = db.Klienci.FirstOrDefault(k => k.Login == klient.Login && k.Password == klient.Password);

            if (user != null)
            {
                TempData["KlientId"] = user.KlientId;
                Session["KlientId"] = user.KlientId;
                Session["Login"] = user.Login;
                Session.Timeout = 20;
                return Redirect("LoggedIn");

            }
            else
            {
                ModelState.AddModelError("", "Login lub hasło jest nieprawidłowe.");
            }

            return View();
        }