Determine NamespacedClassFinder#finder method after rails config

[s-mage]

Hi there!

You're making a great gem, thank you very much.

I've found a bug while profiling a thing in our app.

Formtastic has two modes of finding classes, one is fast and another fetches new files in development. It works in dev mode all the time because use_const_defined? is called when Rails.application is still nil.

As a result, finding input classes sometimes takes half of the time Formtastic works.

This PR fixes this by determining the finder method on the first calling, which likely happens after the app initialization.

Bonus: spec for the class tested changed cache_classes config value, which, I suppose, was used before eager_load.

Testing

I don't know how to make a spec for the case, but here is how I do it manually:

1. config.eager_load = !!ENV["EAGER_LOAD"] # config/environments/development.rb
2. EAGER_LOAD=true rails c
3. For master:

show-source Formtastic::NamespacedClassFinder#finder # for pry
Formtastic::NamespacedClassFinder.new([]).method(:finder).source_location # would work everywhere

For this PR:

Formtastic::NamespacedClassFinder.send(:finder_method)

Workaround

Just in case the fix won't get to master or will be released only for v4. I've added a workaround to reach the same effect

# config/initializers/formtastic_namespaced_class_finder_patch.rb
if Rails.configuration.eager_load
  class Formtastic::NamespacedClassFinder
    private
      def finder(class_name)
        find_with_const_defined(class_name)
      end
  end
end

[mikz]

Makes sense and it looks good. The failing tests were fixed in a different PR. Thanks for this!