Set correct content-type for ActivityPub JSON (#4592)

spec/controllers/activitypub/outboxes_controller_spec.rb

@@ -15,5 +15,9 @@ RSpec.describe ActivityPub::OutboxesController, type: :controller do
     it 'returns http success' do
       expect(response).to have_http_status(:success)
     end
+
+    it 'returns application/activity+json' do
+      expect(response.content_type).to eq 'application/activity+json'
+    end
   end
 end