Add controller tests for switch_locale

This way it'll be easier to change it while checking we haven't broken
existing behavior.

While writing the tests, I noticed we were sometimes storing a symbol in
the session while sometimes we were storing a string. So we're adding a
`to_s` call so we always store a string in the session.

app/controllers/application_controller.rb

@@ -46,7 +46,7 @@ class ApplicationController < ActionController::Base
         current_user.update(locale: locale)
       end
 
-      session[:locale] = locale
+      session[:locale] = locale.to_s
       I18n.with_locale(locale, &action)
     end
 

app/controllers/management/base_controller.rb

@@ -41,10 +41,10 @@ class Management::BaseController < ActionController::Base
 
     def switch_locale(&action)
       if params[:locale] && I18n.available_locales.include?(params[:locale].to_sym)
-        session[:locale] = params[:locale]
+        session[:locale] = params[:locale].to_s
       end
 
-      session[:locale] ||= I18n.default_locale
+      session[:locale] ||= I18n.default_locale.to_s
 
       I18n.with_locale(session[:locale], &action)
     end

app/controllers/subscriptions_controller.rb

@@ -32,7 +32,7 @@ class SubscriptionsController < ApplicationController
 
     def set_user_locale(&action)
       if params[:locale].blank?
-        session[:locale] = I18n.available_locales.find { |locale| locale == @user.locale&.to_sym }
+        session[:locale] = I18n.available_locales.find { |locale| locale == @user.locale&.to_sym }.to_s
       end
       I18n.with_locale(session[:locale], &action)
     end