[Modify] Polish it

Author: sta

websocket-sharp/Net/HttpListenerResponse.cs

@@ -960,8 +960,10 @@ public void SetCookie (Cookie cookie)
  if (cookie == null)
  throw new ArgumentNullException ("cookie");
 
- if (!canAddOrUpdate (cookie))
- throw new ArgumentException ("Cannot be replaced.", "cookie");
+ if (!canAddOrUpdate (cookie)) {
+ var msg = "It cannot be replaced.";
+ throw new ArgumentException (msg, "cookie");
+ }
 
  Cookies.Add (cookie);
  }