Refactor Cache-Control and Vary definitions (#24347)

spec/controllers/custom_css_controller_spec.rb

@@ -6,9 +6,25 @@ describe CustomCssController do
   render_views
 
   describe 'GET #show' do
-    it 'returns http success' do
+    before do
       get :show
+    end
+
+    it 'returns http success' do
       expect(response).to have_http_status(200)
     end
+
+    it 'returns public cache control header' do
+      expect(response.headers['Cache-Control']).to include('public')
+    end
+
+    it 'does not set cookies' do
+      expect(response.cookies).to be_empty
+      expect(response.headers['Set-Cookies']).to be_nil
+    end
+
+    it 'does not set sessions' do
+      expect(session).to be_empty
+    end
   end
 end