Limit lookups to specific classes via :only

Seems straightforward.  The ticket mentioned limiting via `klass < mod` but I would think `klass <= mod` an implemented accordingly...
Can clarify each case here. Pass `only: MyClass` and it's limited to instances of that class or its subclasses. Pass `only: [ MyClass, OtherClass ]` and it's limited to instances of those classes and their subclasses. Pass `only: [ SomeModule ]` and it's limited to classes that include that module.