Improve unit test coverage of all Flag classes

[sappenin]

E.g., see CredentialFlagsTest.java, which covers some (but not all) flag scenarios. We need to ensure that every Flag class has a unit test that covers:

1. All flag values
2. JSON serialization/deserialization of each flag value.
3. JSON serialization/deserialization of empty flags.
4. JSON serialization/deserialization of unset flags.

This issue was instigated by the Batch work where serialization of empty/unset BatchFlags triggered unhandled behavior.

[sappenin]

Some test coverage was improved as part of #653 and #690. However, another audit should be performed to verify JSON coverage.