Added constants to fully comply to standard websocket

[hktonylee]

No description provided.

[nathanboktae]

I really don't want to maintain a typescript definition. Moreover it shouldn't matter, as one place may instantiate it knowing it is a RobustWebSocket but all other consumers treat it as a normal WebSocket instance and do not know the difference. That is also why the static constants were not put on RobustWebSocket. Not interested in taking this PR.

[hktonylee]

That's fine to skip the Typescript definition. But the standard web socket has constants in the prototype. That means anyone can access the constants in both WebSocket.CLOSED and (new WebSocket()).CLOSED. Please check here.

In order to make it fully transparent, it is necessary to add the constants.

[nathanboktae]

Oh I was not aware they were on the prototype too. If.you can scope the PR down to that we can take it.

[hktonylee]

I have force-updated the branch. Please check ;-)