Fix obsolete respond_with_bip usage

Since we removed the `best_in_place` gem, this method doesn't exist
anymore. We're replacing it with what the method actually does.

Note the test doesn't check the poll is correctly updated. We could add
a `visit proposal_dashboard_polls_path(proposal)` before checking the
"Show results" field, but then we would enter a race condition between
this request and the AJAX request. A proper solution would be to provide
actual feedback to the user so they know the poll has been updated, and
then checking that feedback is present in the tests.

app/controllers/dashboard/polls_controller.rb

@@ -35,10 +35,10 @@ class Dashboard::PollsController < Dashboard::BaseController
       if poll.update(poll_params)
         format.html { redirect_to proposal_dashboard_polls_path(proposal),
                                   notice: t("flash.actions.update.poll") }
-        format.json { respond_with_bip(poll) }
+        format.json { head :no_content }
       else
         format.html { render :edit }
-        format.json { respond_with_bip(poll) }
+        format.json { render json: poll.errors.full_messages, status: :unprocessable_entity }
       end
     end
   end

spec/features/dashboard/polls_spec.rb

@@ -197,6 +197,19 @@ feature "Polls" do
     end
   end
 
+  scenario "Enable and disable results", :js do
+    create(:poll, related: proposal)
+
+    visit proposal_dashboard_polls_path(proposal)
+    check "Show results"
+
+    expect(find_field("Show results")).to be_checked
+
+    uncheck "Show results"
+
+    expect(find_field("Show results")).not_to be_checked
+  end
+
   scenario "Poll card" do
     poll = create(:poll, :expired, related: proposal)