Further preparation for Rails 6 (#15916)

* Use ActiveRecord::Result#to_ary instead of deprecated to_hash

They do the same thing, and to_hash has been removed from Rails 6.1

* Explicitly name polymorphic indexes to workaround a bug in Rails 6.1

cf. https://github.com/rails/rails/issues/41693

* Fix incorrect usage of “foreign_key” in migration script

* Use `ActiveModel::Errors#delete` instead of deprecated clear method

* Fix link headers tests on Rails 6.1

Rails 6.1 adds values to the Link header by default, thus it is not a
LinkHeader object anymore. Fix the test to parse the Link header instead
of assuming it is a LinkHeader.

db/migrate/20171119172437_create_admin_action_logs.rb

@@ -3,7 +3,7 @@ class CreateAdminActionLogs < ActiveRecord::Migration[5.1]
     create_table :admin_action_logs do |t|
       t.belongs_to :account, foreign_key: { on_delete: :cascade }
       t.string :action, null: false, default: ''
-      t.references :target, polymorphic: true
+      t.references :target, polymorphic: true, index: { name: 'index_admin_action_logs_on_target_type_and_target_id' }
       t.text :recorded_changes, null: false, default: ''
 
       t.timestamps