Correctly check permissions in locales controller

We were using `authorize_resource`, passing it an unnamed parameter.
When that happens, CanCanCan only checks permissions to read that
resource. But, in this case, we want to check the permission to update
that resource before the `update` action.

Most of the time, it doesn't really matter, but, for example, in our
demo we're going to restrict the locales configuration so locales cannot
be updated on the main tenant (but they can be updated on other
tenants).

app/controllers/admin/locales_controller.rb

@@ -1,6 +1,6 @@
 class Admin::LocalesController < Admin::BaseController
   before_action :set_locales_settings
-  authorize_resource :locales_settings
+  authorize_resource instance_name: :locales_settings, class: "Setting::LocalesSettings"
 
   def show
   end

spec/controllers/admin/locales_controller_spec.rb

@@ -0,0 +1,17 @@
+require "rails_helper"
+
+describe Admin::LocalesController do
+  describe "PATCH update" do
+    it "checks permissions to update locales settings" do
+      user = create(:administrator).user
+      restricted_ability = user.ability.tap { |ability| ability.cannot :update, Setting::LocalesSettings }
+
+      sign_in user
+      allow(controller).to receive(:current_ability).and_return(restricted_ability)
+      patch :update, params: { setting_locales_settings: { default: :es, enabled: [:en, :fr] }}
+
+      expect(response).to redirect_to "/"
+      expect(Setting.default_locale).to eq :en
+    end
+  end
+end