1.2.0-incubating-RC2

This RC2 fixes a recently discovered bug identified by Issue #292 . It was fixed with PR #293, which contained other documentation and code cleanup unrelated to the bug. The specific files changed relating to the bug:

• main/.../datasketches/tuple/Union.java
• test/.../datasketches/tuple/adouble/UpdatableSketchWithDoubleSummaryTest.java