public function appendContent($key, $value)
    {
        if ($this->content === null) {
            $this->content = array();
        }

        if (!is_array($this->content)) {
            throw new \InvalidArgumentException('Cannot append response content: Current content is not an array !');
        }

        $this->content[(string) $key] = $value;

        return $this;
    }