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

spec/system/admin/poll/questions/answers/answers_spec.rb

@@ -58,4 +58,20 @@ describe "Answers" do
 
     expect("Another title").to appear_before("New title")
   end
+
+  scenario "Reorder", :js do
+    question = create(:poll_question)
+    create(:poll_question_answer, question: question, title: "First", given_order: 1)
+    create(:poll_question_answer, question: question, title: "Last", given_order: 2)
+
+    visit admin_question_path(question)
+
+    within("tbody.sortable") do
+      expect("First").to appear_before("Last")
+
+      find("tr", text: "Last").drag_to(find("tr", text: "First"))
+
+      expect("Last").to appear_before("First")
+    end
+  end
 end