Locale in i18nfield

```!ruby

article1 = Article.new(title: {en: 'Article title', fr: 'Titre article'})
article2 = Article.new(title: {en: 'Other article title', fr: 'Titre autre article'})
resultset = [article1, article2]

decorated = Locomotive::Decorators::I18nDecorator.new(article1, :fr)

decorated.title # => Titre article

decorated.current_locale = :en
decorated.title # => Article title

decorated_result = Locomotive::Decorators::I18nDecorator.decorate(resultset, :fr)

decorated_result.last.title # => Titre autre article

decorated.title[:fr]=''
decorated.current_locale = :fr

# default callback behaviour : first locale
decorated.title # 'Other article title'

# custom callback behaviour
decorated.on_empty_callback = Proc.new { |fields, locale| "Missing translation #{locale}" }

decorated.title # 'Missing translation fr'

decorated.on_empty_callback = Proc.new { |fields, locale| nil }

decorated.title # nil
```