Make `Single.concat(Publisher)` behavior consistent on cancel

[idelpivnitskiy]

Motivation:

Behavior of Single.concat(Publisher) is different compare to all other concat variants: Single.concat(Completable), Single.concat(Single), Completable.concat(Completable), Completable.concat(Single), Completable.concat(Publisher), Publisher.concat(Completable), Publisher.concat(Single), Publisher.concat(Publisher). It does not subscribe to the next source to propagate cancellation if onSuccess is delivered after cancel.

Modifications:

• Make tests for all concat valiants consistent in regards to cancellation;
• Modify Single.concat(Publisher) behavior to subscribe in case onSuccess is delivered after cancel;

Result:

Behavior of Single.concat(Publisher) is consistent with all other concat variants.

[idelpivnitskiy]

Decided to open a PR now as it shouldn't cause merge conflicts with #2381.

[idelpivnitskiy]

@Scottmitch rebased after your changes, ready for review

[Scottmitch]

@idelpivnitskiy - should we close this or do you still think we need this?