Use `eager_load` instead of `cache_classes` in `NamespacedClassFinder`

I get an error in my tests with the custom inputs in my forms on a Rails 4.1.8 app with Formtastic 3.1.0 when using the new finders.
In my test environment, where I have `eager_load = false` and `cache_classes = true`, the classes are not found (`ActionView::Template::Error: Unable to find input class ...Input`).

In `formtastic/helpers/input_helper.rb`, `eager_load` is used to determine if `const_defined?` should be used.
But in the new finders, `cache_classes` is used.
Is there any reason for this change ?