protected void invokeSetter(Method method, String fieldName, Object value) {
        if (value == null) {
            objects.remove(fieldName);
        } else {
            objects.put(fieldName, value);
        }
    }
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