Fix taking associated failure app from the scope in the given env.

There is a delegator to get failure app, introduced in 4629bee and tuned
in 24b26026. The latter commit introduced a bit of logic, however, no
tests are included into commit. Needless to say this resulted in a
broken code.

The point is that `env["warden.options"][:scope]` returns a string.
However, `Devise.mappings` is a hash with symbol keys.

Adding tests and converting scope to symbol here.

lib/devise/delegator.rb

@@ -8,9 +8,9 @@ module Devise
     def failure_app(env)
       app = env["warden.options"] &&
         (scope = env["warden.options"][:scope]) &&
-        Devise.mappings[scope].failure_app
+        Devise.mappings[scope.to_sym].failure_app
 
       app || Devise::FailureApp
     end
   end
-end
+end

test/delegator_test.rb

@@ -0,0 +1,19 @@
+require 'test_helper'
+
+class DelegatorTest < ActiveSupport::TestCase
+  def delegator
+    Devise::Delegator.new
+  end
+
+  test 'failure_app returns default failure app if no warden options in env' do
+    assert_equal Devise::FailureApp, delegator.failure_app({})
+  end
+
+  test 'failure_app returns default failure app if no scope in warden options' do
+    assert_equal Devise::FailureApp, delegator.failure_app({"warden.options" => {}})
+  end
+
+  test 'failure_app returns associated failure app by scope in the given environment' do
+    assert_kind_of Proc, delegator.failure_app({"warden.options" => {:scope => "manager"}})
+  end
+end