Fixed a bug when a vocabulary factory returns int values

Seems that Plone 4 introduced a problem with **vocabularies with int values**. We found this on a customer application migrated from Plone 3 to Plone 4.2.

When Vocabulary method is used, it always return a `DisplayList`, not a `IntDisplayList`.

I added some test for this, but I think there's still something wrong in Archetypes for this. Right now you pass a `content_instance` to `Vocabulary` method, it still return a DisplayList (see tests).

I fear that the same bug is there on Plone 4.3 and Archetypes 1.9. To be checked.