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.

spec/controllers/subscriptions_controller_spec.rb

@@ -19,5 +19,13 @@ describe SubscriptionsController do
       expect(response).to redirect_to "/"
       expect(flash[:alert]).to eq "No tienes permiso para acceder a esta página."
     end
+
+    it "uses the user locale where there's no locale in the parameters" do
+      create(:user, locale: "es", subscriptions_token: "mytoken")
+
+      get :edit, params: { token: "mytoken" }
+
+      expect(session[:locale]).to eq "es"
+    end
   end
 end