Using RFC 5261 to patch an XML document using Laravel Guzzle

I'm looking to patch a XML file using the RFC 5261 standard. However I'm having trouble with submitting the information. I'm using the Laravel Guzzle client to do so however all I can retrieve is an unsupported media type.

  $this->http->patch(".../$id", [
                'headers' => [
                    'Content-Type' => 'application/xml'
                ],
                'body' => ["
                    <diff>
                        <replace sel='Registration/Status'>Cancelled</replace>
                    </diff>
                "],
            ]);

I've tried different variations of the Content Type such as text/xml but had no luck either. Is my formatting incorrect? I've also tried using form_params instead of the body but that didnt seem to be the issue