2 new tests for OnSubscribeToObservableFuture

1) Creating an Observable from an already-cancelled Future should emit nothing (equivalent to Observable.never())
   (it currently emits onError(CancellationException)
2) Creating an Observable from a Future then unsubscribeing immediately before Future.get()
   unblocks should be an Observable.never().  This test already passes.
Modify OnSubscribeToObservableFuture to return early if unsubscribe without emitting anything
Merge pull request #1291 from mattrjacobs/check-unsubscribe-within-observable-from-future

Check unsubscribe within observable from future