Fix side effect of no_dereference on GenericReferenceField

[bagerard]

Similarly to how #1892 fixed the issue for ReferenceField, this PR fixes the issue for GenericReferenceField that is suffering from the exact same issue. See below:

class Organization(me.Document):
    name = me.StringField()
    
class User(me.Document):
    name = me.StringField()
    organization = me.GenericReferenceField()

organization = Organization(name='Github').save()
User(name='John', organization=organization).save()

user = User.objects.first()   # fetch an instance before the no_dereference call
user_no_derf = User.objects.no_dereference().first() # alters the _auto_dereference of the class field (i.e the ReferenceField descriptor) 

print(user_no_derf.organization)    # {'_cls': 'Organization', '_ref': DBRef('organization', ObjectId('5baaa27e4ec5dc49f0ffc86a'))} (This is fine)
print(user.organization)    # Should be an Organization instance but is 
                            # {'_cls': 'Organization', '_ref': DBRef('organization', ObjectId('5baaa27e4ec5dc49f0ffc86a'))}