8.3.37

Features:

• d8c3ac6 Feature: Support DebugSpy in Release Builds (reactivemarbles#798) @dwcullop

Fixes:

• 49ad041 Fix ObservableSpy null mismatch issues (reactivemarbles#803) @dwcullop

Other:

• ff8b94a Implemented a variety of utilities for testing the core correctness of RX operators and streams, including notification sequencing and synchronization. (reactivemarbles#797) @JakenVeina
• 6680627 Sealed more internal classes (reactivemarbles#799) @dwcullop