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

`preserved_as_paper` has a schema level default that causes
hasattr(aq_base(mail), 'preserved_as_paper' 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.
Merge pull request #503 from 4teamwork/lg-set-proper-default-for-preserved-as-paper-for-mails

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