Optionalsession: any
Optional value attached to the resulting client as client.session. Defaults to {} if omitted.
Optionalprotocol: string
Optionally override subprotocol selection. When omitted, the first mutually-supported subprotocol (in server preference order) is used. Passing a protocol that the client did not advertise rejects the connection with HTTP 400.
Call this function to accept the client, causing the server to emit a
'client'event.