dynamicfield with reference value raise 'field not exist' error when query

• Environments
  python version:3.6.8
  mongoengine version:0.15.3.
• operation reproduct

from mongoengine import *
class Person(Document):
    name=StringField()
class SomeTest(DynamicDocument):
    name=StringField()
connect() #suppose a default server
p=Person(name='Jack').save()
t=SomeTest(name='a')
t.person=p
t.save()
SomeTest.objects #this will raise 'field not exist' error

• reason
  In 'base/document.py', the '__expand_dynamic_values' function did not construct document
  object correctly. It call "cls(**value)" direclty when "is_dict and '_cls' in value". But the test document is saved as something like :

{
  "_id" : {
    "$oid" : "5dd60794dde73f1a64ddf052"
  },
  "name" : "a",
  "person" : {
    "_ref" : {
      "$ref" : "person",
      "$id" : {
        "$oid" : "5dd5fcfadde73f15c4548073"
      }
    },
    "_cls" : "Person"
  }
}

When assigned a document object to the dynamic field, it is saved as a dict with keys ['_ref','_cls'], where '_ref' repsenting a DBRef object.
When query the dynamic document, the '__expand_dynamic_values' function trys to construct a document object with 'cls(**{'_ref':...,'_cls':...])', so it raise error as '_ref' is not a field of the cls.
The latest codes of the function still keep same as version 0.15.3.

• suggest
  Modify the '__expand_dynamic_values' function as:

        # If the value is a dict with '_cls' in it, turn it into a document
        is_dict = isinstance(value, dict)
        if is_dict and '_cls' in value:
            cls = get_document(value['_cls'])
            # my patch to consturct document correctly
            if '_ref' in value: # and isinstance(value['_ref'],DBRef) ?
                return cls.objects(id=value['_ref'].id).first()
            # my patch to consturct document correctly
            return cls(**value)

The patch works in my environment.
Or modify the assignment process of dynamic field. I have not read the codes, so may have some approaches more better.

Start in base/document.py at __expand_dynamic_values and reproduce the failure with the Python example in the issue, focusing on dynamic fields containing a referenced document. Verify how the stored _ref and _cls values should be expanded, then confirm that SomeTest.objects succeeds and returns the referenced Person without a field-not-found error.