added .disable_inheritance method for the simple fetch exclusive object class

For use in test:
```python
class A(Document):
    x = IntField()
    y = IntField()

    meta = {'allow_inheritance': True}

class B(A):
    z = IntField()

A.drop_collection()

A(x=10, y=20).save()
A(x=15, y=30).save()
B(x=20, y=40).save()
B(x=30, y=50).save()

for obj in A.objects.disable_inheritance():
    self.assertEqual(obj.__class__, A)