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.
Fixes#11884.
