Use notifyUrl not returnUrl for callback

Omnipay supports a notifyUrl for callbacks and a returnUrl for the browser. This patch causes the notifyUrl to be used for callbacks by securePay

Author: eileenmcnaughton

src/Message/DirectPostAuthorizeRequest.php

@@ -22,7 +22,7 @@ public function getData()
         $data['EPS_TIMESTAMP'] = gmdate('YmdHis');
         $data['EPS_FINGERPRINT'] = $this->generateFingerprint($data);
         $data['EPS_RESULTURL'] = $this->getReturnUrl();
-        $data['EPS_CALLBACKURL'] = $this->getReturnUrl();
+        $data['EPS_CALLBACKURL'] = $this->getNotifyUrl();
         $data['EPS_REDIRECT'] = 'TRUE';
         $data['EPS_CURRENCY'] = $this->getCurrency();