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/activitypub/collections_controller_spec.rb

@@ -6,6 +6,21 @@ RSpec.describe ActivityPub::CollectionsController, type: :controller do
   let!(:account) { Fabricate(:account) }
   let(:remote_account) { nil }
 
+  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
+
   before do
     allow(controller).to receive(:signed_request_account).and_return(remote_account)
 
@@ -31,9 +46,7 @@ RSpec.describe ActivityPub::CollectionsController, 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 orderedItems with pinned statuses' do
           json = body_as_json
@@ -58,9 +71,7 @@ RSpec.describe ActivityPub::CollectionsController, 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 orderedItems with pinned statuses' do
             json = body_as_json