When #deliver_now is called all translations within the
generated email will be looked up for the current I18n
locale.
I18n.locale = ‘de’
mail.deliver_now # Generates german email, correct
In #enqueue_delivery the locale was not considered and
the resulting job uses the default locale.
I18n.locale = ‘de’
mail.deliver_later # Generate english email, incorrect
In order to achieve a consistent behaviour the current locale
is now always passed to `ActionMailer::DeliveryJob`.
Action Mailer tests weren't able to run in isolation without the bundle
exec prefix since we were requiring gems before requiring abstract_unit.
We don't need the `gem` call thus and the require_relative since the
test directory should be present in the load path when we run any test.
