private function drain(
        StreamInterface $source,
        StreamInterface $sink,
        $contentLength
    ) {
        // If a content-length header is provided, then stop reading once
        // that number of bytes has been read. This can prevent infinitely
        // reading from a stream when dealing with servers that do not honor
        // Connection: Close headers.
        Psr7\copy_to_stream(
            $source,
            $sink,
            (strlen($contentLength) > 0 && (int) $contentLength > 0) ? (int) $contentLength : -1
        );

        $sink->seek(0);
        $source->close();

        return $sink;
    }