.to_json() didn't skip default values when query with .only().

here is the sample:

from mongoengine import Document, ListField, StringField, connect


class T(Document):
    a = StringField()
    b = StringField(default="B")
    c = ListField()

connect()
t1 = T(a='a')
t1.save()

t2 = T.objects(id=t1.id).only('a')
t2.to_json()
t2[0].to_json()

expect output:
>>> '"_id": {"$oid": "object id"}, "a": "a"}'
actual output:
>>> '"_id": {"$oid": "object id"}, "a": "a", "b": "B", "c": []}'

Maybe we should keep __only_fields(in created) as a default fields for to_mongo() ?
And I think to_json() should add fields kwargs to propagate for to_mongo().

Start by tracing the .only() query result through to_json() and to_mongo(), focusing on how __only_fields is created and propagated. Reproduce the sample, then verify that serialization omits the default b and c fields while retaining _id and a.