Fix issue 1662 digikam login

include/ws_protocols/rest_encoder.php

@@ -142,8 +142,7 @@ class PwgRestEncoder extends PwgResponseEncoder
 {
   function encodeResponse($response)
   {
-    $respClass = strtolower( @get_class($response) );
-    if ($respClass=='pwgerror')
+    if ($response instanceof PwgError)
     {
       $ret = '<?xml version="1.0"?>
 <rsp stat="fail">