Fix NPE when iterable is null
Removed unused import and variables.
Merge pull request #1751 from akarnokd/Remove_unused

Removed unused imports and variables.
Merge pull request #1750 from zsxwing/OnSubscribeFromIterabl-NPE

Fix NPE when iterable is null
Added missing headers, removed unnecessary casts
Merge pull request #1752 from akarnokd/Cleanup_Headers_n_Casts

Added missing headers, removed unnecessary casts
OnSubscribeRefCount with Synchronous Support

Fixes https://github.com/ReactiveX/RxJava/issues/1688
OnSubscribeRefCount UnitTests

Additional tests on top of those started by @davidmoten
Merge pull request #1755 from benjchristensen/issue1688

OnSubscribeRefCount with Synchronous Support
Cleanup: added missing annotations, removed unnecesary annotations
Merge pull request #1757 from akarnokd/Cleanup_Override_n_SuppressWarnings

Cleanup: added missing annotations, removed unnecesary annotations
Combine RefCount Tests
Merge pull request #1763 from benjchristensen/cleanup-refCount-tests

Combine RefCount Tests
0.20.6
set producer to allow back pressure signals to flow between parent and child
Unhandled errors go to UncaughtExceptionHandler

Rather than swallowing/logging errors, ScheduledAction now delivers them
to the UncaughtExceptionHandler for the executing Thread. This gives
client applications control over the handling of errors that occur off
of the calling Thread.
Merge pull request #1766 from loganj/uncaught

Unhandled errors go to UncaughtExceptionHandler
Merge pull request #1765 from neerajrj/onErrorResumeBackpressure

backpressure support in onErrorResumeNext* operators
ExecutorScheduler delivers uncaught exceptions

Instead of swallowing unhandled errors, ExecutorScheduler delivers them
to the executing thread's UncaughtExceptionHandler.

This addresses the same issue as ReactiveX/RxJava#1682, but for
ExecutorScheduler which does not used ScheduledAction.
OnErrorResume Backpressure Tests