Make it possible to unit-test extensions to Devise::SessionsController

This time with tests

app/controllers/devise/sessions_controller.rb

@@ -11,6 +11,17 @@ class Devise::SessionsController < ApplicationController
   # POST /resource/sign_in
   def create
     resource = warden.authenticate!(:scope => resource_name, :recall => "#{controller_path}#new")
+
+    # In the running app, the previous line would actually cause this method to
+    # exit by throwing `:warden` if the authentication failed. Unfortunately,
+    # this doesn't happen in the Rails test environment if you have included the
+    # Devise::TestHelpers (see `Devise::TestHelpers::TestWarden#authenticate!`),
+    # which makes it difficult to unit test extensions to this controller. Since
+    # the resource is nil if authentication fails, just short-circuit the method
+    # in that case. This should not affect the running app.
+    
+    return if resource.nil?
+
     set_flash_message(:notice, :signed_in) if is_navigational_format?
     sign_in(resource_name, resource)
     respond_with resource, :location => redirect_location(resource_name, resource)