Missing assert in test_base.py

[gitdb/gitdb/test/test\_base.py at master · gitpython-developers/gitdb](https://github.com/gitpython-developers/gitdb/blob/master/gitdb/test/test_base.py#L95)

```
istream.size = s * 2
istream.size == s * 2
```

`istream.size == s * 2` doesn't do anything. It should probably be `assert istream.size == s * 2`.