Add supportUpdate request

Author: pilot

src/Omnipay/Common/AbstractGateway.php

@@ -189,6 +189,16 @@ public function supportsUnstore()
         return method_exists($this, 'unstore');
     }
 
+    /**
+     * Supports Update
+     *
+     * @return boolean True if this gateway supports the update() method
+     */
+    public function supportsUpdate()
+    {
+        return method_exists($this, 'update');
+    }
+
     /**
      * Create and initialize a request object using existing parameters from this gateway
      */