df.fillna inplace bug 
This code:

```
a = df()
a['f'] = [None, None]
a['s'] = [None, None]
a['t'] = [None, None]
a.loc[:,['f','s']] = a.loc[:,['f','s']].fillna(-1) #works
a.loc[:,'f'].fillna(-1, inplace=True) #also works
a.loc[:,['f','s']].fillna(-1, inplace=True) #doesn't
```

It's not the biggest error, but still.
