Proper way to send (POST) xml with guzzle 6

This is what worked for me on Guzzle 6:

// configure options
$options = [
    'headers' => [
        'Content-Type' => 'text/xml; charset=UTF8',
    ],
    'body' => $xml,
];

$response = $client->request('POST', $url, $options);

After some experiments, I have figured it out. Here is my solution in case someone reaches a dead end.

$request = new Request(
    'POST', 
    $uri,
    ['Content-Type' => 'text/xml; charset=UTF8'],
    $xml
);