update(add_to_set) on a ListField(CachedReferenceField) only includes the _id

Modifying existing CachedReferenceField test code to test a ListField(CachedReferenceField()) using doc.update(add_to_set__animal=animal_instance) results in the _id being in the list. None of the cached fields made it.

from mongoengine import *
from mongoengine.connection import get_db
from mongoengine.base import (BaseDict, BaseField, EmbeddedDocumentList,
                              _document_registry)



class Owner(EmbeddedDocument):
    name = StringField()
    tags = ListField(StringField())

class Animal(Document):
    name = StringField()
    tag = StringField()

    owner = EmbeddedDocumentField(Owner)

class Ocorrence(Document):
    person = StringField()
    animal = ListField(CachedReferenceField(
        Animal, fields=['tag', 'owner.tags']))

Animal.drop_collection()
Ocorrence.drop_collection()

a = Animal(name="Leopard", tag="heavy",
           owner=Owner(tags=['cool', 'funny'],
                       name="Wilson Júnior")
           )
a2 = Animal(name="Tiger", tag="too heavy",
           owner=Owner(tags=['cool2', 'funny2'],
                       name="Wilson")
           )
a.save()
a2.save()
o = Ocorrence(person="teste 2")
o.save()
o.update(add_to_set__animal=a)
o.update(add_to_set__animal=a2)

# doing it this way works as documented
#o = Ocorrence(person="teste 2", animal=[a,a2])
#o.save()

Start by running the reproduction using the Owner, Animal, and Ocorrence definitions shown in the issue, comparing update(add_to_set__animal=animal_instance) with saving a populated list. Trace how the CachedReferenceField values are serialized during the add_to_set update. Done means both cached fields, including nested owner.tags, remain available in the stored list and the regression is covered by a test.