Prepare Mastodon for Rails 6 (#15911)

* Fix misuse of foreign_type

* Fix use of removed "add_template_helper"

* Use response.media_type instead of response.content_type in tests

* Fix CSV export controller test on Rails 6

Rails 6 sets a "filename*" field in the Content-Disposition header to
explicitly encode the filename as UTF-8.

This changes checks the first part of the Content-Disposition header so
it matches in both Rails 5 and Rails 6.

* Fix emoji formatting with Rails 6

* Make emoji output more idiomatic and robust

* Switch from redis-rails gem to built-in Rails redis cache storage

1 files changed, 1 insertions, 1 deletions

diff --git a/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb b/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb
index 88f4554c2..d373f56bd 100644
--- a/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb
+++ b/spec/controllers/activitypub/followers_synchronizations_controller_spec.rb
@@ -40,7 +40,7 @@ RSpec.describe ActivityPub::FollowersSynchronizationsController, type: :controll
       end
 
       it 'returns application/activity+json' do
-        expect(response.content_type).to eq 'application/activity+json'
+        expect(response.media_type).to eq 'application/activity+json'
       end
 
       it 'returns orderedItems with followers from example.com' do