OperatorMulticast.connect(connection) should not return null

[davidmoten]

See discussion in #2774

Changes include

• fix for ConnectableObservable.connect() returns a null subscription if already connected #2774 just for OperatorMulticast (will do OperatorPublish in another PR once have sorted this one out)
• made fields private that had default visibility
• renamed OnSubscribeMulticastTest to OperatorMulticastTest
• addressed a possible race condition that could provoke an IAE when Observable.subscribe(sub) is called with sub=null (unsubscription occurs between L144 and L145 on old code).

[akarnokd]

I try to avoid private because the compiler generates accessor methods which slows down the interpreter phase and makes an extra hop while debugging methods.

[davidmoten]

Thanks @akarnokd, sharp-eyed as ever. I used a final AtomicReference to help with the == this check, is this the idiom you'd use in this situation?

[akarnokd]

Thanks!