[FIX] Fetch json key from item serializer if empty collection is passed to collection serializer and each_searializer is specified.

test/fixtures/poro.rb

@@ -183,6 +183,12 @@ PaginatedSerializer = Class.new(ActiveModel::Serializer::CollectionSerializer) d
   end
 end
 
+MessagesSerializer = Class.new(ActiveModel::Serializer) do
+  def json_key
+    'messages'
+  end
+end
+
 AlternateBlogSerializer = Class.new(ActiveModel::Serializer) do
   attribute :id
   attribute :name, key: :title