* Fix g_mime_message_get_date ownership txfr GI annotation

The message owns the date (it's effectively a property), so don't
transfer ownership.

Fixes a critical in vala when using this object.

See GNOME/vala#986



* Add missing nullable GI annotations to GMimeMessage

These help out bindings for languages that care about nullability,
like vala.

Co-authored-by: Rico Tzschichholz <ricotz@ubuntu.com>