The disposable was not added, so it was never called (Swift beta 3)

Hello.

I do not know if it is a bug from current beta version of Swift or the `Atomic` implementation, but the value inside `_disposables` was not equal to `ds` (that is a copy, not a reference), so `d` was added to `ds` and not to `_disposables` value; therefore the `d` disposable was never called.

Congrat for the great work you are doing!