Don't use cached user if session has been reset (#79)

This is a bit of an edge case, but if the user is logged in and then the
session is reset (or at least its `current_user` value is reset) and
`RpiAuth::Controllers::CurrentUser#current_user` is called within the
*same* request, then previously it would have returned the user "cached"
in the `@current_user`. Now it will return `nil` which seems less
surprising.

This was highlighted by [this example][1] in code-club-frontend. In that
case the example passes, because that app has [its own implementation of
`#current_user`][2]. However, when I added a similar example in
experience-cs where we're using
`RpiAuth::Controllers::CurrentUser#current_user`, the example failed.

I've added a rather contrived example by adding a new
`HomeController#reset_user` action to the dummy app used in the specs.
However, hopefully it helps explain a bit about why the change is
necessary.

It looks as if the only two RPF repos that use
`RpiAuth::Controllers::CurrentUser` are experience-ai [here][3] and
[here][4] and [experience-cs][5]. I'm pretty confident this change won't
affect either of those apps, so this can probably be a minor version
bump.

I think a possible alternative to making this change would be to provide
a `#reset_user` method which could both reset the session *and* reset
the ivar-cached user. However, given that the code-club-frontend
implementation already uses this approach, I think that probably makes
more sense for now.

[1]:
https://github.com/RaspberryPiFoundation/code-club-frontend/blob/901f2200e926741fd170858286d64f1dd49994b4/spec/requests/refresh_credentials_spec.rb#L90
[2]:
https://github.com/RaspberryPiFoundation/code-club-frontend/blob/901f2200e926741fd170858286d64f1dd49994b4/app/controllers/concerns/authentication_concern.rb#L6-L11
[3]:
https://github.com/RaspberryPiFoundation/experience-ai/blob/ad729cefe060fcc0f0c345b3297247069fe55867/app/controllers/application_controller.rb#L6
[4]:
https://github.com/RaspberryPiFoundation/experience-ai/blob/ad729cefe060fcc0f0c345b3297247069fe55867/app/controllers/api/v1/zipped_resources_controller.rb#L6
[5]:
https://github.com/RaspberryPiFoundation/experience-cs/blob/33fa4756371fa3a550f32ac7613130e0925c17d3/app/controllers/application_controller.rb#L4

Author: floehopper

lib/rpi_auth/controllers/current_user.rb

@@ -10,8 +10,8 @@ module CurrentUser
       end
 
       def current_user
-        return @current_user if @current_user
         return nil unless session[:current_user]
+        return @current_user if @current_user
 
         @current_user = RpiAuth.user_model.new(session[:current_user])
       end

spec/dummy/app/controllers/home_controller.rb

@@ -1,4 +1,10 @@
 class HomeController < ApplicationController
   def show
   end
+
+  def reset_user
+    current_user
+    reset_session
+    render :show
+  end
 end

spec/dummy/config/routes.rb

@@ -1,6 +1,7 @@
 Rails.application.routes.draw do
   # For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html
   root to: 'home#show'
+  get '/reset-user', to: 'home#reset_user'
 
   resource :session, only: %i[create]
 

spec/dummy/spec/requests/auth_request_spec.rb

@@ -182,6 +182,15 @@
         expect(session.id).not_to eq previous_id
       end
 
+      it 'does not use cached user if session is reset' do
+        post '/auth/rpi'
+        follow_redirect!
+
+        get reset_user_path
+
+        expect(response.body).to include('Log in')
+      end
+
       context 'when session_keys_to_persist is set' do
         let(:session_keys_to_persist) { 'foo' }