[Modify] Polish it

websocket-sharp/Net/HttpListenerResponse.cs

@@ -622,8 +622,10 @@ namespace WebSocketSharp.Net
           return;
         }
 
-        if (value.Length == 0)
+        if (value.Length == 0) {
-          throw new ArgumentException ("An empty string.", "value");
+          var msg = "An empty string.";
+          throw new ArgumentException (msg, "value");
+        }
 
         Uri uri;
         if (!Uri.TryCreate (value, UriKind.Absolute, out uri))