Improve set_locale

We discard session[:locale] as valid locale when it is no longer include in
the :available_locales

spec/system/localization_spec.rb

@@ -88,4 +88,22 @@ describe "Localization" do
       end
     end
   end
+
+  scenario "uses default locale when session locale has disappeared" do
+    default_locales = I18n.available_locales
+
+    visit root_path(locale: :es)
+
+    expect(page).to have_content "Entrar"
+
+    begin
+      I18n.available_locales = default_locales - [:es]
+
+      visit root_path
+
+      expect(page).to have_content "Sign in"
+    ensure
+      I18n.available_locales = default_locales
+    end
+  end
 end