#589: Fix schema not including base class for records with subclasses

[MichalFoksa]

It is draft, do not review yet.

[cowtowncoder]

Sounds good. Was about to suggest that due to RecordVisitor changes (addition of a Field) should go against 2.x not 2.19 but since it's private field maybe it can be considered internal (non-API) change.

[MichalFoksa]

I have problem to choose most obvious handling of duplicates in unionSchemas (inside if (subTypes != null && !subTypes.isEmpty()) condition).

1. when I use HashSet then _typeSchema must be added into union last.

2. when I use ArrayList, then I have to call unionSchemas .contains(subTypeSchema) before adding subTypeSchema into unionSchemas. However ArrayList.contains() is only object reference-equality check.

3. If reference-equality check is enough for unionSchemas , then IdentityHashMap can be used to create a Set (Set<Schema> unionSchemas = Collections.newSetFromMap(new IdentityHashMap<>()) ) and we have simple code.