def index(options = {}, &block)
      respond_with(collection, options,  &block) if stale?(collection, last_modified: collection.maximum(:updated_at))
    end
def index(options = {}, &block)
      respond_with(collection, options,  &block) if stale?(collection, last_modified: collection.maximum(:updated_at))
    end
index action, this set a collection of objects to an instance variable
 which can be accessed from the view using the collection helper method.
 The instance variable name is a pluralization of the model name defined
 in the restful macro.