fillna bug on ordered string data

OK

```
x = Series([NaN,1.,NaN,3.,NaN],['a','b','c','d','e'], dtype=object)
x.fillna(method='pad')
```

Not OK

```
x = Series([NaN,1.,NaN,3.,NaN],['z','a','b','c','d'], dtype=object)
x.fillna(method='pad')
```
