code: TypeSerializer<?> getSubclassSerializer(Class<?> subclass) {
  		TypeSerializer<?> result = subclassSerializerCache.get(subclass);
  		if (result == null) {
  			result = createSubclassSerializer(subclass);
  			subclassSerializerCache.put(subclass, result);
  		}
  		return result;
  	}
docs: Fetches cached serializer for a non-registered subclass;
  also creates the serializer if it doesn't exist yet.
  
  This method is also exposed to package-private access
  for testing purposes.