Bring password_digest back.

This method is part of the protected API and is used by custom
encryption engines (like `devise-encryptable`) to hook the custom
encryption logic in the models.

Fixes #2730

test/models/database_authenticatable_test.rb

@@ -93,6 +93,11 @@ class DatabaseAuthenticatableTest < ActiveSupport::TestCase
     assert_present user.encrypted_password
   end
 
+  test 'should support custom encryption methods' do
+    user = UserWithCustomEncryption.new(:password => '654321')
+    assert_equal user.encrypted_password, '123456'
+  end
+
   test 'allow authenticatable_salt to work even with nil encrypted password' do
     user = User.new
     user.encrypted_password = nil

test/test_models.rb

@@ -12,6 +12,13 @@ class UserWithValidation < User
   validates_presence_of :username
 end
 
+class UserWithCustomEncryption < User
+  protected
+  def password_digest(password)
+    password.reverse
+  end
+end
+
 class UserWithVirtualAttributes < User
   devise :case_insensitive_keys => [ :email, :email_confirmation ]
   validates :email, :presence => true, :confirmation => {:on => :create}