Fix: DeliveryDate missing from request

ShippingDate isn't a parameter, according to PostNL's Documentation:
- https://developer.postnl.nl/docs/#/http/api-endpoints/checkout-delivery-options/deliverydate/calculate-shipping-date

DeliveryDate parameter only accepts dd-mm-yyyy format, which is the following pattern:
- ^[0-3]\d-[0-1]\d-[1-2]\d{3}$
So I formatted the DateTimeInterface (returned by the getDeliveryDate method) to oblidge by that format.

Author: gsdkkr

src/Service/DeliveryDateService.php

@@ -464,7 +464,7 @@ public function buildGetSentDateRequestREST(GetSentDateRequest $getSentDate)
 
         $sentDate = $getSentDate->getGetSentDate();
         $query = [
-            'ShippingDate' => $sentDate->getDeliveryDate(),
+            'DeliveryDate' => $sentDate->getDeliveryDate()->format('d-m-Y'),
         ];
         $query['CountryCode'] = $sentDate->getCountryCode();
         if ($duration = $sentDate->getShippingDuration()) {