fix: use $response instance instead of static Response::addHeader() in StartSessionMiddleware

Response::addHeader() is an instance method, not static. The static
call silently fails in PHP 8.x (caught by the Error handler), so
the session Set-Cookie header is never added to the response.

Fixes #297

Author: Ibrahim BinAlshikh

WebFiori/Framework/Middleware/StartSessionMiddleware.php

@@ -26,7 +26,7 @@ public function after(Request $request, Response $response) {
             $sessionsCookiesHeaders = SessionsManager::getCookiesHeaders();
 
             foreach ($sessionsCookiesHeaders as $headerVal) {
-                Response::addHeader('set-cookie', $headerVal);
+                $response->addHeader('set-cookie', $headerVal);
             }
         } catch (Error $exc) {
         }