Fixed ActionDisposable not disposing correctly, as the underlying value ...

...was never reset to nil.

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Performed a minimally invasive surgery to fix the bug. I'm not so sure about the ?! construct, but it was the least changes required. Please note though, that with this, I'm not leveraging your fancy thread-safe modify behavior. Nevertheless before I got rid of the derived state (`disposed:Bool` via `return _action == nil` instead of actual state similar to `SerialDisposable`) I thought I'd supply this fix first.
I don't think this should have a default value. When would you want to initialize an `ActionDisposable` that does nothing?