$addToSet operator is not adding None values correctly

[dongxinEric]

When using $addToSet operator to add a None value to an array who has a deeply nested None value, DocLayer thinks the None value is duplicated and thus throws it away.

For example this reproducible case:

coll.remove()
coll.insert_one({'_id':1, 'B': [{'a': {'b': {'c': {'d':None}}}}]})
coll.update({'_id':1}, {'$addToSet': {u'B': None}})

The document is NOT updated correctly.

[dongxinEric]

My gut feeling is that this is caused by the same root bug that also caused #135