title: Controller SessionStateBehavior is ReadOnly and I can update Session Variable
body: <p>I expect that if controller has attribute <code>SessionStateBehavior.ReadOnly</code> then I can't change session variables inside this controller
  but I can change values.</p>
  
  <p>I try this code</p>
  
  <pre><code> [SessionState(System.Web.SessionState.SessionStateBehavior.ReadOnly)]
      public class GLobalController : Controller
      {
        public  ActionResult Index()
          {
              Session["xxx"] = DateTime.Now.ToString();
              return View();
          }
  </code></pre>