public function getAcceptableContentTypes()
    {
        if ($this->_contentTypes === null) {
            if ($this->headers->get('Accept') !== null) {
                $this->_contentTypes = $this->parseAcceptHeader($this->headers->get('Accept'));
            } else {
                $this->_contentTypes = [];
            }
        }

        return $this->_contentTypes;
    }