jsupdater should not tinker with the flash, fixes #4507

Closes #4507

Merge request studip/studip!3300

app/controllers/jsupdater.php

@@ -30,6 +30,9 @@ class JsupdaterController extends AuthenticatedController
     {
         parent::before_filter($action, $args);
 
+        // Keep all things in flash, this controller show tinker with those values
+        $this->flash->keep();
+
         // Check for a valid logged in user (only when an ajax request occurs)
         if (Request::isXhr() && (!is_object($GLOBALS['user']) || $GLOBALS['user']->id === 'nobody')) {
             $this->response->set_status(403);