Fix lack of thread safety in -[RACSerialDisposable disposable]

Addresses #1455

I couldn't come up with a way to do it using `OSAtomicXXX`, but I'd be happy to change it if somebody can suggest how.
To date, we've shied away from using `AsyncBlock` tests, primarily because it makes things confusing and hard to read. Can this test be refactored to use Expecta's `will` matcher instead?