Accept switch_to_locale param in LocaleController

Previously we accepted only the :locale param in LocaleController#set.

solidus_i18n, however, only accepts the :switch_to_locale param. This is
because solidus_i18n uses the :locale param as part of it's path
(ex. /en/products).

For better compatiblity with solidus_i18n, and so that we can merge most
of it's functionality, we should accept either param here (and probably
should prefer :switch_to_locale).

frontend/app/controllers/spree/locale_controller.rb

@@ -3,8 +3,12 @@
 module Spree
  class LocaleController < Spree::StoreController
  def set
- if params[:locale] && I18n.available_locales.map(&:to_s).include?(params[:locale])
- session[:locale] = I18n.locale = params[:locale]
+ available_locales = Spree.i18n_available_locales
+ requested_locale = params[:switch_to_locale] || params[:locale]
+
+ if requested_locale && available_locales.map(&:to_s).include?(requested_locale)
+ session[:locale] = requested_locale
+ I18n.locale = requested_locale
  flash.notice = t('spree.locale_changed')
  else
  flash[:error] = t('spree.locale_not_changed')