Add regression test to #8907

railties/test/application/middleware/static_test.rb

+# encoding: utf-8
+require 'isolation/abstract_unit'
+require 'rack/test'
+
+module ApplicationTests
+  class MiddlewareStaticTest < ActiveSupport::TestCase
+    include ActiveSupport::Testing::Isolation
+    include Rack::Test::Methods
+
+    def setup
+      build_app
+      FileUtils.rm_rf "#{app_path}/config/environments"
+    end
+
+    def teardown
+      teardown_app
+    end
+
+    # Regression test to #8907
+    # See https://github.com/rails/rails/commit/9cc82b77196d21a5c7021f6dca59ab9b2b158a45#commitcomment-2416514
+    test "doesn't set Cache-Control header when it is nil" do
+      app_file "public/foo.html", 'static'
+
+      require "#{app_path}/config/environment"
+
+      get 'foo'
+
+      assert_not last_response.headers.has_key?('Cache-Control'), "Cache-Control should not be set"
+    end
+  end
+end