Calling .complete() unconditionnally from the json encoder is unsafe
since on entity creation validation:

* an eid may have been drawn (hence even .has_eid() would not help)
  while processing form data in the edit controller

* a ValidationError may have been raised and the entity-creating
  transaction rollbacked

This leads to a crash on the return path from the validation to the
browser, where the json_dumps((status, args, entity)) call will
stumble upon the .complete() call which will fail because the entity
does not (any more) exist in the database.