Before f359f267 (Use Distributed Cache for Session, 2021-07-19)
when we were using RAM cache for sessions, it was possible to store
collections (eg. dicts or lists) with reference to ERP5 documents in
session, but after this change, using such collections was refused
with error:

    TypeError: Can't pickle objects in acquisition wrappers.

This change restore the possibility of using collections, by being
more clever when we store and retrieve collections