`in_bulk` queries do not include a `_cls` attribute, so objects that inherit from the same base class can unexpectedly be returned

[eengoron]

Right now, when using the in_bulk method, a _cls is not included in the query.

So, if you have documents like:

class Parent(Document):
    ...

class ChildOne(Parent):
    ...

class ChildTwo(Parent):
    ...

And then you try to do:

ChildTwo.objects.in_bulk(["PK of a ChildOne object"])

a ChildOne object will be returned even though you're using the ChildTwo document. This happens because a _cls is not included in the in_bulk query, so it just looks through the entire collection.