controllers: refactor only param includes.

Add extra includes needed by the `only` param inside `respond_with`.

app/controllers/favorite_groups_controller.rb

@@ -4,7 +4,9 @@ class FavoriteGroupsController < ApplicationController
 
   def index
     params[:search][:creator_id] ||= params[:user_id]
-    @favorite_groups = FavoriteGroup.paginated_search(params).includes(model_includes(params))
+    @favorite_groups = FavoriteGroup.paginated_search(params)
+    @favorite_groups = @favorite_groups.includes(:creator) if request.format.html?
+
     respond_with(@favorite_groups)
   end
 
@@ -61,14 +63,6 @@ class FavoriteGroupsController < ApplicationController
 
   private
 
-  def default_includes(params)
-    if ["json", "xml"].include?(params[:format])
-      []
-    else
-      [:creator]
-    end
-  end
-
   def check_write_privilege(favgroup)
     raise User::PrivilegeError unless favgroup.editable_by?(CurrentUser.user)
   end