controllers: refactor only param includes.

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

app/controllers/dtext_links_controller.rb

@@ -2,17 +2,9 @@ class DtextLinksController < ApplicationController
   respond_to :html, :xml, :json
 
   def index
-    @dtext_links = DtextLink.paginated_search(params).includes(model_includes(params))
+    @dtext_links = DtextLink.paginated_search(params)
+    @dtext_links = @dtext_links.includes(:model) if request.format.html?
+
     respond_with(@dtext_links)
   end
-
-  private
-
-  def default_includes(params)
-    if ["json", "xml"].include?(params[:format])
-      []
-    else
-      [:model]
-    end
-  end
 end