Add attribute_for_database attribute method

I've been reported an issue on enum from friends, they want a way to get
mapped value, but currently there is no reliable way to get that value.

If a record is loaded from database, `attribute_before_type_cast` works
for that. but the attribute is changed by user, the attribute method
won't work for that.

```ruby
book = Book.new(status: "published")

# returns "published", but what we really want is 2.
book.status_before_type_cast
```

So I propose to add `attribute_for_database` attribute method, it
consistently returns mapped value for enum.

activerecord/lib/active_record/attribute_methods/before_type_cast.rb

@@ -29,7 +29,7 @@ module ActiveRecord
       extend ActiveSupport::Concern
 
       included do
-        attribute_method_suffix "_before_type_cast"
+        attribute_method_suffix "_before_type_cast", "_for_database"
         attribute_method_suffix "_came_from_user?"
       end
 
@@ -72,6 +72,10 @@ module ActiveRecord
           @attributes[attr_name].value_before_type_cast
         end
 
+        def attribute_for_database(attr_name)
+          @attributes[attr_name].value_for_database
+        end
+
         def attribute_came_from_user?(attr_name)
           @attributes[attr_name].came_from_user?
         end

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -54,7 +54,7 @@ module ActiveRecord
         end
 
         module ClassMethods
-          ID_ATTRIBUTE_METHODS = %w(id id= id? id_before_type_cast id_was id_in_database).to_set
+          ID_ATTRIBUTE_METHODS = %w(id id= id? id_before_type_cast id_was id_in_database id_for_database).to_set
 
           def instance_method_already_implemented?(method_name)
             super || primary_key && ID_ATTRIBUTE_METHODS.include?(method_name)

activerecord/test/cases/enum_test.rb

@@ -290,7 +290,7 @@ class EnumTest < ActiveRecord::TestCase
     assert_predicate Book.illustrator_visibility_invisible.create, :illustrator_visibility_invisible?
   end
 
-  test "_before_type_cast" do
+  test "attribute_before_type_cast" do
     assert_equal 2, @book.status_before_type_cast
     assert_equal "published", @book.status
 
@@ -300,6 +300,16 @@ class EnumTest < ActiveRecord::TestCase
     assert_equal "published", @book.status
   end
 
+  test "attribute_for_database" do
+    assert_equal 2, @book.status_for_database
+    assert_equal "published", @book.status
+
+    @book.status = "published"
+
+    assert_equal 2, @book.status_for_database
+    assert_equal "published", @book.status
+  end
+
   test "invalid definition values raise an ArgumentError" do
     e = assert_raises(ArgumentError) do
       Class.new(ActiveRecord::Base) do