Failing test for disposal not cutting off sent values
RACDisposable unit tests
Add RACDisposable.disposed
Allow RACDisposable to be created without a block
Skip unnecessary disposables in recursive scheduling
Remove `disposed` variable in normal scheduling
Skip unnecessary disposable in replay subject subscription
Wrap all signal subscribers in a passthrough object

When the disposable for _that specific subscription_ is disposed, no
more events will be passed to the inner subscriber. The inner
subscriber may, however, remain alive (as it should).
Merge branch 'master' into subscription-not-disposing
Merge pull request #582 from ReactiveCocoa/subscription-not-disposing

Fix subscribers receiving events even after subscription disposal