Broken nunique on Series group by
The following code works in 0.16.2 and not in latest master:

``` python
data = pd.DataFrame(
    [[100, 1, 'Alice'],
     [200, 2, 'Bob'],
     [300, 3, 'Charlie'],
     [-400, 4, 'Dan'],
     [500, 5, 'Edith']],
    columns=['amount', 'id', 'name']
)

expected = data.groupby(['id', 'amount'])['name'].nunique()
```

Going to bisect this today unless someone beats me to it.
