Add #model_name instance method to ActiveModel::Naming

This pull request addresses #15428 and [discussion on the Rails Core mailing list](https://groups.google.com/forum/#!topic/rubyonrails-core/ThSaXw9y1F8).
Good point, but the problem is that `ActiveModel::Naming` is supposed to be `extend`ed, not `include`d. So we can't just add it to `Naming`. You can still do something like:

```ruby
def self.extended(base)
  base.include ActiveModel::Naming::InstanceMethods
end

module InstanceMethods
  def model_name
    self.class.model_name
  end
end
```

But it doesn't sound right to me since `InstanceMethods` in `ActiveSupport::Concern` was deprecated, we are not using `ActiveSupport::Concern` in this module, though.