Calling stub.reset() makes stub no longer override return value

```python
import sinon
import os
os.system("cd") # returns 0
stub = sinon.stub(os, "system").returns(5)
os.system("cd") # returns 5
stub.callCount # returns 1
stub.reset()
stub.callCount # returns 0
os.system("cd") # returns 0, which is wrong! should return None
stub.returns(5)
os.system("cd") # returns 0, which is wrong! should return 5
```