Follows the same pattern as controllers and jobs. Exceptions raised in
delivery jobs (enqueued by `#deliver_later`) are also delegated to the
mailer's rescue_from handlers, so you can handle the DeserializationError
raised by delivery jobs:
```ruby
class MyMailer < ApplicationMailer
rescue_from ActiveJob::DeserializationError do
…
end
```
ActiveSupport::Rescuable polish:
* Add the `rescue_with_handler` class method so exceptions may be
handled at the class level without requiring an instance.
* Rationalize `exception.cause` handling. If no handler matches the
exception, fall back to the handler that matches its cause.
* Handle exceptions raised elsewhere. Pass `object: …` to execute
the `rescue_from` handler (e.g. a method call or a block to
instance_exec) against a different object. Defaults to `self`.
They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:
```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```
The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*
This raises an exception now.
Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.
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.
