[Modify] Add RSV check

websocket-sharp/WebSocket.cs

@@ -744,6 +744,10 @@ namespace WebSocketSharp
                  ? "A data frame has been received while receiving the fragmented data."
                  : frame.IsCompressed && _compression == CompressionMethod.None
                    ? "A compressed frame is without an available decompression method."
+                   : frame.Rsv2 == Rsv.On
+                     ? "The RSV2 of a frame is non-zero without any negotiation for it."
+                     : frame.Rsv3 == Rsv.On
+                       ? "The RSV3 of a frame is non-zero without any negotiation for it."
                        : null;
     }