Transaction::getForExchange - cast order_id to string

[alcohol]

If not explicitly cast to string a SimpleXMLElement is passed instead.

This results (in my clients codebase at least) in the following error being returned by the API:

Paynl\\Error\\Api(code: 0):  - TransactionId is not a string at <projectroot>/vendor/paynl/sdk/src/Api/Api.php:155)

Client is running version 1.15.20.

I haven't actually verified what the SDK sends to be honest so I can't say for sure if this issue is valid. But the following work-around did solve the issue for my client:

- $transaction = Transaction::getForExchange();
+ $input = file_get_contents('php://input');
+ $xml = simplexml_load_string($input);
+ $orderId = (string) $xml->order_id;
+ $transaction = Transaction::get($orderId);