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/management/base_controller_spec.rb

@@ -1,6 +1,16 @@
 require "rails_helper"
 
 describe Management::BaseController do
+  before { session[:manager] = double }
+
+  controller do
+    skip_authorization_check
+
+    def index
+      render plain: I18n.locale
+    end
+  end
+
   describe "managed_user" do
     it "returns existent user with session document info if present" do
       session[:document_type] = "1"
@@ -21,4 +31,38 @@ describe Management::BaseController do
       expect(managed_user.document_number).to eq "333333333E"
     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
+  end
 end