autosave error messages cannot be translated (i18n)

This seems to be similar to #3147 ActiveRecord::Error#full_message broken when used with I18n and autosave asso....

ActiveRecord::Schema.define do
  create_table(:owners) { |o| o.string :username }
  create_table(:items)  { |i| i.string :property }
end

class Owner < ActiveRecord::Base
  has_one :item, :autosave => true
  validates_presence_of :username
end

class Item < ActiveRecord::Base
  validates_presence_of :property
end

I18n.backend.store_translations :en, :"activerecord.attributes.item.property" => "Testing1"

owner = Owner.new
owner.build_item
owner.valid?
owner.errors.full_messages

item = Item.create
item.errors.full_messages

This code above will result in
["Item property can't be blank", "Username can't be blank"] and
["Testing1 can't be blank"]

The first output is not as expected. It should be like ["Testing1 can't be blank", "Username can't be blank"].

With the following patch, it works as expected.

Maybe there is a better fix, but this works for my project.