Use `eager_load` instead of `cache_classes`

In `formtastic/helpers/input_helper.rb`, `eager_load` is used to
determine if `const_defined?` should be used.
With `cache_classes = true`, classes are still not loaded at startup, so
some inputs may not be loaded.
Merge pull request #1100 from gdott9/namespaced_class_finder

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