* Template lookup now respect default locale and I18n fallbacks.

  Given the following templates:

      mailer/demo.html.erb
      mailer/demo.en.html.erb
      mailer/demo.pt.html.erb

  Before this change for a locale that doesn't have its related file the `mailer/demo.html.erb` will
  be rendered even if `en` is the default locale.

  Now `mailer/demo.en.html.erb` has precedence over the file without locale.

  Also, it is possible to give a fallback.

      mailer/demo.pt.html.erb
      mailer/demo.pt-BR.html.erb

  So if the locale is `pt-PT`, `mailer/demo.pt.html.erb` will be rendered given the right I18n
  fallback configuration.

  *Rafael Mendonça França*

Please check [4-2-stable](https://github.com/rails/rails/blob/4-2-stable/actionmailer/CHANGELOG.md) for previous changes.