[Modify] Polish it

websocket-sharp/Net/HttpListenerResponse.cs

@@ -1084,8 +1084,10 @@ namespace WebSocketSharp.Net
       if (url == null)
         throw new ArgumentNullException ("url");
 
-      if (url.Length == 0)
+      if (url.Length == 0) {
-        throw new ArgumentException ("An empty string.", "url");
+        var msg = "An empty string.";
+        throw new ArgumentException (msg, "url");
+      }
 
       Uri uri;
       if (!Uri.TryCreate (url, UriKind.Absolute, out uri))