Potential regression in handling of the _cls field

Hi,

Consider the following example:

from mongoengine import *

connect('test', host='mongodb://localhost/test')

class Page(Document):
    title = StringField()
    meta = {'allow_inheritance': True}

class DatedPage(Page):
    date = DateTimeField()

page = DatedPage()
page.save()

assert '_cls' not in (key for key in page)

With mongoengine 0.8.x the _cls key was not returned during iteration, which I think is the correct behaviour.

Start with the Python reproduction in the issue and inspect MongoEngine's document iteration and handling of the internal _cls field for inherited documents. Confirm the behavior against MongoDB, then add regression coverage showing that iterating a saved DatedPage does not expose _cls; done when the assertion passes.