Added a unit test to demonstrate regression in Observable.mergeDelayError
Fix the failing Observable.mergeDelayError synchronous error unit test
Fix compose generics
Fix Rx serialization bug in takeUntil again
Fix the race condition in BufferUntilSubscriber
Use EmptyObserver instead of Subscribers.empty()

onError should be ignored after unsubscribe is called.
Merge pull request #1714 from benjchristensen/merge-20

Merge 0.20.5 into 1.x