public function withContentType($contentType)
    {
        $contentType = $this->normalizeContentType($contentType);

        $contentType = explode(';', $contentType, 2);

        $this->type = $contentType[0];

        $contentType[0] .= ';' . (isset($contentType[1]) ? $contentType[1] : ' charset=utf-8');

        return $this->withHeader('Content-Type', $contentType[0]);
    }