public function content($content = null)
    {
        if ($content !== null) {
            if (!is_scalar($content) && !is_callable([$content, '__toString'])) {
                throw new ResponseException('Response content must be a scalar or object with __toString() method "' . $this->getType($content) . '" given.');
            }

            $this->content = (string) $content;
        }

        return $this->content;
    }