Compatibility with Redmine 2.5.1

Redmine 2.5 adds new tests ; especially this one
```
Mailer_test.rb => "test_should_log_delivery_errors_when_raise_delivery_errors_is_false"
```
which is broken by the redmine_notified plugin.

I wonder what's the best way to avoid breaking core tests.
Should we just not throw the custom error if Rails.env is test as I suggest in this pull request ?
Or should we initialize message_id if it's nil and Rails.env is test ?

Any better idea ?