AIMRefund fix PHP < 5.5 bugs

Author: mikealmond

src/Message/AIMRefundRequest.php

@@ -17,9 +17,12 @@ public function getData()
 
         $data['x_trans_id'] = $this->getTransactionReference();
         $data['x_card_num'] = $this->getCard()->getNumber();
-        if (!empty($this->getCard()->getExpiryMonth())) {
+
+        $expiryMonth = $this->getCard()->getExpiryMonth();
+        if (!empty($expiryMonth)) {
             $data['x_exp_date'] = $this->getCard()->getExpiryDate('my');
         }
+
         $data['x_amount'] = $this->getAmount();
 
         return $data;