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/application_controller_spec.rb

@@ -1,6 +1,14 @@
 require "rails_helper"
 
 describe ApplicationController do
+  controller do
+    skip_authorization_check
+
+    def index
+      render plain: I18n.locale
+    end
+  end
+
   describe "#current_budget" do
     it "returns the last budget that is not in draft phase" do
       create(:budget, :finished,  created_at: 2.years.ago, name: "Old")
@@ -12,4 +20,50 @@ describe ApplicationController do
       expect(budget.name).to eq("Current")
     end
   end
+
+  describe "#switch_locale" do
+    it "uses the default locale by default" do
+      get :index
+
+      expect(response.body).to eq "en"
+    end
+
+    it "uses the locale in the parameters when it's there" do
+      get :index, params: { locale: :es }
+
+      expect(response.body).to eq "es"
+    end
+
+    it "uses the locale in the session if there are no parameters" do
+      get :index, params: { locale: :es }
+
+      expect(response.body).to eq "es"
+
+      get :index
+
+      expect(response.body).to eq "es"
+    end
+
+    it "uses the locale in the parameters even when it's in the session" do
+      get :index
+
+      expect(response.body).to eq "en"
+
+      get :index, params: { locale: :es }
+
+      expect(response.body).to eq "es"
+    end
+
+    context "authenticated user" do
+      let(:user) { create(:user) }
+      before { sign_in(user) }
+
+      it "updates the prefered locale when it's in the parameters" do
+        get :index, params: { locale: :es }
+
+        expect(user.reload.locale).to eq "es"
+        expect(response.body).to eq "es"
+      end
+    end
+  end
 end