Improve Coverage for HashTable, Fix Clone

[oleibman]

Add unit tests to cover all of HashTable. I was hoping to do this without source changes, but this class does require a deep clone, and, as the new unit tests revealed, the existing code did not fill the bill - it cloned objects, but not arrays which contained objects, and all the object variables in this class are arrays which can contain objects.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?