title: HttpContext getting null when returning from WebPaymentAPI
body: <p>I'm using a webpayment api solution on my website.
  When the API asks my OnPost() function and returning to my back url my HttpContext seems to be replaced by an other HttpContext.</p>
  
  <p>I can't get my HttpContext.Session back, all datas are removed.</p>
  
  <p>(I'm already using services.addSession() &amp; app.useSession())</p>
  
  <p>Code of my OnPost Method from the VerifPoints.cshtml page</p>
  
  <pre><code>public void OnPost()
      {
          Utilisateur = Utilisateur.Current(HttpContext); //Calling a HttpContext.Session.GetInt32("UserID")   /!\ THIS IS NULL /!\
  
  
          string json = get_web_content("URLOFMYWEBPAYMENTAPI" + Request.Form["code"]);
  
          dynamic array = JsonConvert.DeserializeObject(json);
  
          if (array.status == "success")
          {
              int montant = array.virtual_currency;
              HttpContext.Session.SetInt32("Pbs", montant);
  
              Response.Redirect("/compte/recharger_points");
          }
          else
          {
              HttpContext.Session.SetInt32("Pbs", 0);
  
              Response.Redirect("/compte/recharger_points");
          }
      }
  </code></pre>