2.2.1 release preparations #6159

Version 2.2.1 - August 23, 2018 (Maven)

API changes

• Pull 6143: Add concatArrayEagerDelayError operator (expose feature).

Bugfixes

• Pull 6145: Fix boundary fusion of concatMap and publish operator.
• Pull 6158: Make Flowable.fromCallable consistent with the other fromCallables.
• Pull 6165: Handle undeliverable error in Completable.fromCallable via RxJavaPlugins.
• Pull 6167: Make observeOn not let worker.dispose() get called prematurely.

Performance improvements

• Pull 6123: Improve Completable.onErrorResumeNext internals.
• Pull 6121: Flowable.onErrorResumeNext improvements.

Documentation changes

JavaDocs

• Pull 6095: Add marbles for Single.timer, Single.defer and Single.toXXX operators.
• Pull 6137: Add marbles for Single.concat operator.
• Pull 6141: Add marble diagrams for various Single operators.
• Pull 6152: Clarify TestObserver.assertValueSet in docs and via tests.
• Pull 6155: Fix marble of Maybe.flatMap events to MaybeSource.

Wiki changes

• Pull 6128: Remove fromEmitter() in wiki.
• Pull 6133: Update _Sidebar.md with new order of topics.
• Pull 6135: Initial clean up for Combining Observables docs.
• Pull 6131: Expand Creating-Observables.md wiki.
• Pull 6134: Update RxJava Android Module documentation.
• Pull 6140: Update Mathematical and Aggregate Operators docs.