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`.