[AIMRefund] Pass credit card expiration date only when an expiration was set

Authorize.Net does not require the expiration date when performing refunds. When no expiration is passed in and a refund request is made, the response comes back with an error saying the card has expired. 

This check makes it so the expiration date is only sent when a valid expiry date is passed in to the request parameters. 

> At least the last four digits of the credit card number (x_card_num) used for the
original, successfully settled transaction are submitted. An expiration date is not
required.

Author: mikealmond

src/Message/AIMRefundRequest.php

@@ -17,7 +17,9 @@ public function getData()
 
         $data['x_trans_id'] = $this->getTransactionReference();
         $data['x_card_num'] = $this->getCard()->getNumber();
-        $data['x_exp_date'] = $this->getCard()->getExpiryDate('my');
+        if (!empty($this->getCard()->getExpiryMonth())) {
+            $data['x_exp_date'] = $this->getCard()->getExpiryDate('my');
+        }
         $data['x_amount'] = $this->getAmount();
 
         return $data;