Support partial refunds.

Author: andris-sevcenko

src/base/Gateway.php

@@ -407,6 +407,15 @@ public function supportsCompletePurchase(): bool
         return $this->gateway()->supportsCompletePurchase();
     }
 
+    /**
+     * @inheritdoc
+     */
+    public function supportsPartialRefund(): bool
+    {
+        // All of the currently implemented Omnipay gateways support partial refunds.
+        return true;
+    }
+
     /**
      * @inheritdoc
      */