refactor: create separate _defineAccessor() method to conditionally move getters/setters from object to object
fix: Fix the _omitProps issue and take getters/setters into account (using _defineAccessor)
refactor: Use _defineAccessor() in _extendWithGetSet as well
refactor: Make model.isNew() to also check for the idAttribute value (to allow singleton resources with { idAttribute: false })