support multiple warden configuration blocks

Changes the behavior of `Devise.warden` such that calling it multiple
times with different blocks will result in a call to each block on
`Devise.configure_warden!` rather than "last block wins". This is
especially used for plugins that wish to extend warden functionality
without clobbering base app configuration or vice versa.

test/devise_test.rb

@@ -3,10 +3,10 @@ require 'test_helper'
 module Devise
   def self.yield_and_restore
     @@warden_configured = nil
-    c, b = @@warden_config, @@warden_config_block
+    c, b = @@warden_config, @@warden_config_blocks
     yield
   ensure
-    @@warden_config, @@warden_config_block = c, b
+    @@warden_config, @@warden_config_blocks = c, b
   end
 end
 
@@ -53,6 +53,23 @@ class DeviseTest < ActiveSupport::TestCase
     end
   end
 
+  test 'warden manager user configuration through multiple blocks' do
+    Devise.yield_and_restore do
+      @first_executed = false
+      @second_executed = false
+      Devise.warden do |config|
+        @first_executed = true
+      end
+      Devise.warden do |config|
+        @second_executed = true
+      end
+
+      Devise.configure_warden!
+      assert @first_executed
+      assert @second_executed
+    end
+  end
+
   test 'add new module using the helper method' do
     assert_nothing_raised(Exception) { Devise.add_module(:coconut) }
     assert_equal 1, Devise::ALL.select { |v| v == :coconut }.size