fix xml ocs response for serializable objects

[sualko]

If an OCS endpoint returns data which extends JsonSerializable the xml serialization should not fail.

Example in Nextcloud 23:

public function index(): DataResponse {
  return new DataResponse(
			"recommendations" => [/* object implementing JsonSerializable */]
		);
}

Will result in:

<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>200</statuscode>
  <message>OK</message>
 </meta>
 <data>
  <recommendations/>
 </data>
</ocs>

The json response ?format=json is showing the list of recommendations as expected.

With the following error message in the logs:

XMLWriter::writeElement() expects parameter 2 to be string, object given at \/opt\/nextcloud\/23\/lib\/private\/AppFramework\/OCS\/BaseResponse.php#148

Discovered in nextcloud/recommendations#458

Edit: Of course this could also be done on the controller side, but I think it's so common and should be handled by the base response.

[ChristophWurst]

👍 🐘

[susnux]

/backport to stable28

[susnux]

Required for #44263