Fix setting default for `preserved_as_paper` for mails in upgrade step.

`preserved_as_paper` has a [schema level default](https://github.com/4teamwork/opengever.core/blob/master/opengever/document/behaviors/metadata.py#L123) that causes
the [check `hasattr(aq_base(mail), 'preserved_as_paper'`](https://github.com/4teamwork/opengever.core/blob/master/opengever/mail/upgrades/to3401.py#L64) to be `True` even though
the mail doesn't have a value for that field. This caused the value
from the default value adapter (registry) not to be set.