Fetch country id from order address instead of country (#792)

OrderAddressInterface does not contain getCountry method
Co-authored-by: Attila Kiss <42297201+cyattilakiss@users.noreply.github.com>

Gateway/Command/PayByMailCommand.php

@@ -157,8 +157,8 @@ class PayByMailCommand implements CommandInterface
 
         // if directory lookup is enabled use the billingadress as countrycode
         if ($countryCode == false) {
-            if (is_object($order->getBillingAddress()) && $order->getBillingAddress()->getCountry() != "") {
-                $countryCode = $order->getBillingAddress()->getCountry();
+            if (is_object($order->getBillingAddress()) && $order->getBillingAddress()->getCountryId() != "") {
+                $countryCode = $order->getBillingAddress()->getCountryId();
             } else {
                 $countryCode = "";
             }