Fix: Handle collection.objects when it's a partial function

collection.objects can be a partial function. When it is, trying to call in_bulk on it was causing error.

Note: I added explicit type check before calling it, but I can change it to be always called (because query set is callable).

Hi, could you clarify in which case collection.objects is a partial and provide a test case ?

Hello,
This issue occurs when the custom queryset manager function takes more than two parameters. In that case, it gets wrapped in a partial, as shown in the snippet from mongoengine/queryset/manager.py:

else:
    queryset = partial(self.get_queryset, owner, queryset)

I'll be adding a test case shortly.