added etags on prepare_for_display items and menuitems

front-api/controllers/item.rb

@@ -1,5 +1,5 @@
 def prepare_items_for_mass_display(items)
-  items.to_json(
+  items_json = items.to_json(
      :except => [:created_at, :current_input_price, :stock, :on_display],
      :include => [
       :unit ,
@@ -7,6 +7,8 @@ def prepare_items_for_mass_display(items)
       :sub_category,
       :brand
   ])
+  etag xxhash(items_json)
+  items_json
 end
 
 def offset_and_limit_invalid?(offset, limit)