failed to close WebSocket: received header with unexpected rsv bits set: true:true:true

What is the error type of this error so I can ignore it?

Here is an example of an error type within this library.

```go
closeErr := new(websocket.CloseError)
if errors.As(err, closeErr) {
...
}
```

`websocket.CloseError` is an error type within this library.

So, what error type is returned when you receive the error `failed to close WebSocket: received header with unexpected rsv bits set: true:true:true`?

Start with the reported close error and the websocket.CloseError example, then trace the library's WebSocket close-error handling to identify the returned error type. Confirm whether that type is suitable for errors.As matching, and document the result so users can reliably ignore this condition.

Written by the indexing model from the issue text.