Wsd.close without a close code does not send a close frame

In

https://github.com/anmonteiro/websocketaf/blob/248a2cb0dcffa51996c3ad7643577dce75d67454/lib/wsd.ml#L62-L72

`Wsd.close ~code:1000` sends a close frame with status 1000, but `Wsd.close` without `~code` appears to just hard-close the WebSocket connection. This might not be unreasonable, but IMO it should be documented, because another reasonable interpretation is that omitting `~code` should cause a default code to be sent.