Fix `JavaType.isEnumType()` to support sub-classes

[cowtowncoder]

(note: follow up for #2457)

So, JavaType.isEnumType() will only detect direct Enum subclasses as being Enums.
It should allow transitive types (needed for sub-classing by extension within Enum declaration), but doing that will break 3 unit tests. Need to investigate why, work around: they all seem to be related to EnumSet which is tricky type.

[cowtowncoder]

2 failures on TestEnumSerialization (and one in TestDefaultForObject) seem to indicate that somehow enum values in EnumSet gets mistakenly treated as a POJO.