Problem disconnecting on final

Reported by @ainonovo, initially here https://github.com/defold/extension-websocket/issues/48

```
local function websocket_disconnect(self)
if self.ws_con ~= nil then
pprint(self.ws_con)
websocket.disconnect(self.ws_con)
end
end

function final(self)
websocket_disconnect(self)
end
```
in console

```
userdata: 0x0a109ad0
ERROR:WEBSOCKET: Failed to setup callback
```

this code in a script file. Maybe I'm disconnecting at the wrong time - but I need to 100% disconnect correctly - otherwise I can't reconnect