private function createParserWith(StreamInterface $body)
    {
        $class = NullParser::class;
        foreach (self::$parsers as $parser => $contentTypes) {
            foreach ($contentTypes as $contentType) {
                if (stripos($this->contentType, $contentType) !== false) {
                    $class = $parser;
                }
            }
        }
        return new $class($body);
    }