CompositeDisposable: `add` shouldn't be ignored when already disposed

[wbinnssmith]

It seems dangerous for a CompositeDisposable to do nothing if another disposable is added after the CompositeDisposable has been disposed.

For instance, Nuclide's UniversalDisposable immediately throws if it's been disposed of itself, and RxJS's Subscription immediately
unsubscribes from the passed subscription.

Throwing seems like a pretty good option in my opinion as I can't think of a good reason to add to an already disposed composite disposable, but maybe there is?