Fix persisting translations with presence validation of translated fields.

If you added additional validations of translated fields to your translation model, you are not able to persist model and its translations because of validation errors.

    class Post::Translation
      validates :title, presence: true
    end

Validation errors appear on empty translation which is built when `:locale` attribute is fetched on `save` (inside `#read_attribute` method).
This pull-request fixes such issues.