doOnEmpty implementation

[davidmoten]

This operator moved to rxjava-extras master branch from ReactiveX/RxJava#3624.

cc/ @thomasnield (author)

• converted to OnSubscribeDoOnEmpty to save allocations associated with lift
• removed done checks because of guarantees of observable contract
• added unit tests, now at 100% coverage

@thomasnield if I've missed something pls let me know, when it looks good to you I'll release to Maven Central.

[thomasnield]

I've never seen this interface implemented over an Operator before, much less wrapped into a Transformer and creating a separateObservable off it. I'm curious so I'll check this out for my own learning, but I trust your judgement more than mine! Thanks for including this operator and asking : )