Nelmio\SecurityBundle\EventListener\ForcedSslListener::onKernelResponse PHP Method

    public function onKernelResponse(FilterResponseEvent $e)
    {
        if (HttpKernelInterface::MASTER_REQUEST !== $e->getRequestType()) {
            return;
        }
        $response = $e->getResponse();
        if (!$response->headers->has('Strict-Transport-Security')) {
            $header = 'max-age=' . $this->hstsMaxAge;
            $header .= $this->hstsSubdomains ? '; includeSubDomains' : '';
            $header .= $this->hstsPreload ? '; preload' : '';
            $response->headers->set('Strict-Transport-Security', $header);
        }
    }