feat(wrapper): differentiate between setWrapperClass and getWrapperClass

Consider the following use case:
1. User does not specify a `wrapperClass` on instantiation.
2. User changes the `wrapperClass` to, say, `flat`.
3. User wants to go back to the default one, and sets an `undefined` wrapper.
4. The `wrapperClass` is still `flat` because it is used as the default one.

See #254.