Added type declarations for refund method of gateways

Author: vuongxuongminh

src/AppInAppGateway.php

@@ -64,7 +64,7 @@ public function queryTransaction(array $options = []): PayQueryStatusRequest
      * {@inheritdoc}
      * @return \Omnipay\Common\Message\RequestInterface|PayRefundRequest
      */
-    public function refund(array $options = [])
+    public function refund(array $options = []): PayRefundRequest
     {
         return $this->createRequest(PayRefundRequest::class, $options);
     }

src/POSGateway.php

@@ -64,7 +64,7 @@ public function queryTransaction(array $options = []): PayQueryStatusRequest
      * {@inheritdoc}
      * @return \Omnipay\Common\Message\RequestInterface|PayRefundRequest
      */
-    public function refund(array $options = [])
+    public function refund(array $options = []): PayRefundRequest
     {
         return $this->createRequest(PayRefundRequest::class, $options);
     }

src/QRCodeGateway.php

@@ -66,7 +66,7 @@ public function queryTransaction(array $options = []): PayQueryStatusRequest
      * {@inheritdoc}
      * @return \Omnipay\Common\Message\RequestInterface|PayRefundRequest
      */
-    public function refund(array $options = [])
+    public function refund(array $options = []): PayRefundRequest
     {
         return $this->createRequest(PayRefundRequest::class, $options);
     }