Rails 4.0 upgrade

couchrest_model 2.0.1 define the method to_key for CastedModel  which returns an string, but actionpack/action_view need that this method returns an array, this fix the issue when edit form_section.

Casted Models don't inherit from CouchRest::Model::Base this cause issue when test for equality because it will use the id from the parent object and this is a problem when have an array of CastedModels.