Keepalive timer handling fixed
Keepalive timer handling fixed

Starting from version 6.0.0, node returns an error ("callback" argument must be a function) when using native keepalives because of setTimeout calling with null timer handler.
theturtle32/WebSocket-Node,theturtle32/WebSocket-Node,theturtle32/WebSocket-Node,theturtle32/WebSocket-Node,theturtle32/WebSocket-Node