Allow warden proxy to change with request in tests

The warden method in the Devise::TestHelpers module adds a Warden proxy
object to the request environment hash under the 'warden' key. Including
this module in your test case registers that method as a callback, which
runs before every test:
https://github.com/plataformatec/devise/blob/v3.4.1/lib/devise/test_helpers.rb#L12

The request object itself is created in a callback added by Rails:
https://github.com/rails/rails/blob/v4.2.0/actionpack/lib/action_controller/test_case.rb#L687

So before each test runs, the Rails callback creates the request object,
and then the Devise callback adds a Warden proxy object to it.

I was using the rspec-retry gem (https://github.com/y310/rspec-retry),
and noticed that my controller specs would always fail whenever they
were retried with this error:

NoMethodError: undefined method `authenticate!' for nil:NilClass

When rspec-retry re-runs a failed test, it runs the setup callbacks
again. The Rails callback creates a new request object, but because of
the memoization that was here before, the Devise callback wouldn't add a
Warden proxy to it, which was causing the error.

With this change, the Warden setup code will still only run once as long
as the request object stays the same, but if it changes a new Warden
proxy will be added to the new request object.

lib/devise/test_helpers.rb

@@ -26,11 +26,11 @@ module Devise
 
     # Quick access to Warden::Proxy.
     def warden #:nodoc:
-      @warden ||= begin
+      @request.env['warden'] ||= begin
         manager = Warden::Manager.new(nil) do |config|
           config.merge! Devise.warden_config
         end
-        @request.env['warden'] = Warden::Proxy.new(@request.env, manager)
+        Warden::Proxy.new(@request.env, manager)
       end
     end
 

test/test_helpers_test.rb

@@ -160,4 +160,19 @@ class TestHelpersTest < ActionController::TestCase
     get :index
     assert_match /User ##{second_user.id}/, @response.body
   end
+
+  test "creates a new warden proxy if the request object has changed" do
+    old_warden_proxy = warden
+    @request = ActionController::TestRequest.new
+    new_warden_proxy = warden
+
+    assert_not_equal old_warden_proxy, new_warden_proxy
+  end
+
+  test "doesn't create a new warden proxy if the request object hasn't changed" do
+    old_warden_proxy = warden
+    new_warden_proxy = warden
+
+    assert_equal old_warden_proxy, new_warden_proxy
+  end
 end