Fix concurrency bug in ScheduledObserver

- found a concurrency bug while working on https://github.com/Netflix/Hystrix/issues/123
- the following code would lock up occasionally due to onCompleted not being delivered:

```java
public class RunTest {
    public static void main(String[] args) {
        System.out.println("Starting test...");

        final ArrayList<String> strings = new ArrayList<String>(200000);

        int num = 10000;
        while (true) {
            long start = System.currentTimeMillis();
            final AtomicInteger count = new AtomicInteger();
            for (int i = 0; i < num; i++) {
                new TestService1(2, 5).toObservable().forEach(new Action1<Integer>() {

                    @Override
                    public void call(Integer v) {
                        count.addAndGet(v);
                    }
                });

                new TestService2("hello").toObservable().forEach(new Action1<String>() {

                    @Override
                    public void call(String v) {
                        strings.add(v);
                    }

                });
            }
            long time = (System.currentTimeMillis() - start);
            long executions = num * 2;
            System.out.println("Time: " + time + "ms for " + executions + " executions (" + (time * 1000) / executions + " microseconds)");
            System.out.println("   Count: " + count);
            System.out.println("   Strings: " + strings.size());
            strings.clear();
        }
    }
}
```

- Also made OperationObserveOn not use ScheduledObserver if the `ImmediateScheduler` is chosen to allow an optimization. I believe this optimization is safe because ScheduledObserver does not require knowledge of a Scheduler (such as for now()) and all we do is emit data to the Observer on a scheduler and if we know it's Immediate we can go direct and skip the enqueuing step. This allows shaving off a noticable number of microseconds per execution in the loop above.