Fix RSS feeds not being cachable (#14368)

* Add tests for some cachable responses

This only covers responses that we should have managed to make cachable
so far. It's not the case of all responses that should be cachable in
the end.

* Fix RSS feeds not being cachable

spec/controllers/accounts_controller_spec.rb

@@ -5,6 +5,21 @@ RSpec.describe AccountsController, type: :controller do
 
   let(:account) { Fabricate(:user).account }
 
+  shared_examples 'cachable response' do
+    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
+
+    it 'returns public Cache-Control header' do
+      expect(response.headers['Cache-Control']).to include 'public'
+    end
+  end
+
   describe 'GET #show' do
     let(:format) { 'html' }
 
@@ -323,9 +338,7 @@ RSpec.describe AccountsController, type: :controller do
           expect(response.content_type).to eq 'application/activity+json'
         end
 
-        it 'returns public Cache-Control header' do
-          expect(response.headers['Cache-Control']).to include 'public'
-        end
+        it_behaves_like 'cachable response'
 
         it 'renders account' do
           json = body_as_json
@@ -343,9 +356,7 @@ RSpec.describe AccountsController, type: :controller do
             expect(response.content_type).to eq 'application/activity+json'
           end
 
-          it 'returns public Cache-Control header' do
-            expect(response.headers['Cache-Control']).to include 'public'
-          end
+          it_behaves_like 'cachable response'
 
           it 'returns Vary header with Signature' do
             expect(response.headers['Vary']).to include 'Signature'
@@ -401,9 +412,7 @@ RSpec.describe AccountsController, type: :controller do
           expect(response.content_type).to eq 'application/activity+json'
         end
 
-        it 'returns public Cache-Control header' do
-          expect(response.headers['Cache-Control']).to include 'public'
-        end
+        it_behaves_like 'cachable response'
 
         it 'renders account' do
           json = body_as_json
@@ -447,9 +456,7 @@ RSpec.describe AccountsController, type: :controller 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_behaves_like 'cachable response'
       end
 
       context do