allow a QuerySetManager to come from a parent class

Example:

```python
import mongoengine as me

class Base(me.Document):
    active = me.BooleanField(default=True)

    @me.queryset_manager
    def objects(klass, queryset):
        return queryset(active=True)



class Child(Base):
    pass

Child.objects.create(active=False)
assert Child.objects.count() == 0
```