private Map<String, Object> getSerializableFields() {
        return new HashMap<String, Object>(objects);
    }
Returns a Multimap from fieldName Strings to their getter Methods.

It expects to be called only from TaskSerDe. Multimap is used inside org.embulk.config.
embulk-deps/src/main/java/org/embulk/deps/config/TaskInvocationHandler.java