Replace render :nothing with head :ok

Using `render :nothing` was deprecated, but we never noticed it because
we didn't have a test for the action using it. In Rails 5.1, it raises
an exception.

Using `head :ok` and adding a test for this scenario solves the issue.

app/controllers/admin/poll/questions/answers_controller.rb

@@ -44,7 +44,7 @@ class Admin::Poll::Questions::AnswersController < Admin::Poll::BaseController
 
   def order_answers
     ::Poll::Question::Answer.order_answers(params[:ordered_list])
-    render nothing: true
+    head :ok
   end
 
   private