2.2.8 release preparations

[akarnokd]

Version 2.2.8 - March 26, 2019 (Maven)

Bugfixes

• Pull 6442: Add missing undeliverable error handling logic for Completable.fromRunnable & fromAction operators.

Documentation changes

• Pull 6432: Improve the docs of CompositeDisposable.
• Pull 6434: Improve subjects and processors package doc.
• Pull 6436: Improve Creating-Observables wiki doc.

Other

• Pull 6433: Make error messages of parameter checks consistent.

[akarnokd]

Closing via v2.2.8.