DictField is auto-dereferenced if storing data containing _cls property

This bug already existed some time ago (https://github.com/MongoEngine/mongoengine/issues/1058) and there was even a bugfix for it (https://github.com/MongoEngine/mongoengine/pull/1088) but I'm still experiencing this issue..

The bugfix doesn't work for me and I think the test case is not written in a correct way. I'm not sure but it looks like the DictField is auto-dereferenced if it's loaded from the database - which never happens in the test case.

I was able to track down the issue to it's root cause, which is here: https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/base/document.py#L705

The _auto_dereference setting for the DictField is just ignored and overridden, which causes strange behaviour when accessing the field. First it looks like it's a normal instance of a Document, then it's a list with just the property names.

Start at mongoengine/base/document.py around line 705 and reproduce the DictField behavior with data containing an _cls property loaded from the database. Review the existing regression test from the fix referenced in issues #1058 and pull request #1088, then ensure the test covers database loading and distinguishes a DictField value from an auto-dereferenced Document.