* Introduce `ActiveModel::NestedAttributes`

Handle assignment of `_attributes`-suffixed values in an ActionView- and

ActionPack-compliant way:

```ruby

class Article

include ActiveModel::Model

include ActiveModel::NestedAttributes

attr_accessor :author

attr_accessor :tags

accepts_nested_attributes_for :author, class_name: Author

accepts_nested_attributes_for :tags, class_name: Tag

end

article = Article.new(

author_attributes: { name: "Pseudo Nym" },

tags_attributes: {

0 => { name: "actionview" },

1 => { name: "actionpack" },

}

)

article.author.name # => "Pseudo Nym"

article.tags.pluck(:name) # => ["actionview", "actionpack"]

```

*Sean Doyle*