[Modify] Polish it

websocket-sharp/WebSocket.cs

@@ -2873,12 +2873,13 @@ namespace WebSocketSharp
           throw new ArgumentException (msg, "code");
         }
       }
-
-      if (!_client && code == CloseStatusCode.MandatoryExtension) {
+      else {
+        if (code == CloseStatusCode.MandatoryExtension) {
           var msg = "MandatoryExtension cannot be used.";
 
           throw new ArgumentException (msg, "code");
         }
+      }
 
       if (reason.IsNullOrEmpty ()) {
         close ((ushort) code, String.Empty);