public function replaceFormKeyPlaceholder($eventObject) {
        if (Mage::registry('replace_form_key')) {
            $esiHelper = Mage::helper('turpentine/esi');
            $response = $eventObject->getResponse();
            $responseBody = $response->getBody();
            $responseBody = str_replace('{{form_key_esi_placeholder}}',
                $esiHelper->buildEsiIncludeFragment(
                    $esiHelper->getFormKeyEsiUrl() ),
                $responseBody);
            $response->setBody($responseBody);
        }
    }