Add and apply HaveHttpStatus rubocop rule

This rule was added in rubocop-rspec 2.12.0. We were already applying it
most of the time.

spec/controllers/admin/api/stats_controller_spec.rb

@@ -7,7 +7,7 @@ describe Admin::Api::StatsController, :admin do
         get :show
 
         expect(response).not_to be_ok
-        expect(response.status).to eq 400
+        expect(response).to have_http_status 400
       end
     end