Test exposing the essence of #91 : when deleting a Map, even if cascade force deletion of entry, nothing happens
Resolves a side effect of #91 : maps created at object creation have wrong ids assignated to their entries, which prevent deletion of those entries.
Fixed #91 by allowing all tuples to be deleted. Strangely, this bug was never seen before.
Consequence of #91 : removing a map entry having a null element could lead to NullPointerException