Add doOnSubscribe for Single

[aaronweihe]

Ref #3740.

I didn't write much unit tests for doOnSubscribe because it internally uses OperatorDoOnSubscribe which is well covered.

[artem-zinnatullin]

Can you please also verify call order with something like:

List<String> callSequence = new ArrayList(2);

single = Single
  .create(subscriber -> {
    callSequence.add("onSubscribe")
    subscriber.onSuccess(1);
  })
  .doOnSubscribe(() -> callSequence.add("doOnSubscribe"));

single.subscribe();

assertEquals(2, callSequence.size());
assertEquals("doOnSubscribe", callSequence.get(0));
assertEquals("onSubscribe", callSequence.get(1));

[aaronweihe]

@artem-zinnatullin That's a good idea! Just added doOnSubscribeShouldInvokeActionBeforeSubscriberSubscribes test.

[artem-zinnatullin]

👍

[akarnokd]

👍

[stevegury]

👍

[artem-zinnatullin]

Thanks, @aaronhe42!