Fix hook and cookie logic

Echo cannot be used in migrated pages. Write needs to be called on cookie to be properly saved. It didn't work and even @PierreRambaud didn't know why.

Author: Hlavtox

ps_googleanalytics.php

@@ -139,11 +139,15 @@ public function hookDisplayAdminOrder()
     {
         $gaTagHandler = new PrestaShop\Module\Ps_Googleanalytics\Handler\GanalyticsJsHandler($this, $this->context);
 
-        echo $gaTagHandler->generate(
+        $output = '';
+        $output = $gaTagHandler->generate(
             $this->context->cookie->__get('ga_admin_refund'),
             true
         );
         $this->context->cookie->__unset('ga_admin_refund');
+        $this->context->cookie->write();
+
+        return $output;
     }
 
     /**