#76 slice data import/export doesn't work

[Pep Turro]

When exporting an slice "to another actionapps"
including the slice content (items), importing it later
does not import the items properly.
The item table gets part of the content, but at least
some crucial fields are not transfered. publish_date
and expiry_date are set to null for all items.
No data is inserted on the content table.