Strange results when slicing objects using ix
Here's a weird one [from SO](http://stackoverflow.com/questions/17020763/pandas-0-10-1-to-0-11-0-is-method/). Bug in 0.11.0 (fixed in dev).

```
accurate_ICB = ['SA EQUITY CFD', 'SA EQUITY', 'SA SSF']
data = {'Classification': ['SA EQUITY CFD', 'bbb', 'SA EQUITY', 'SA SSF', 'aaa'],
    'Random': [1,2,3,4,5],
    'X': ['correct', 'wrong','correct', 'correct','wrong']}
df =pd.DataFrame(data)
x = df[~df.Classification.isin(accurate_ICB)]
df.ix[x.index,'X'] = df['Classification']
print df
```

In pandas 0.11.0 it produces:

```
  Classification  Random        X
0  SA EQUITY CFD       1  correct
1            bbb       2      bbb
2      SA EQUITY       3  correct
3         SA SSF       4  correct
4            aaa       5      bbb
```

In pandas 0.11.1dev it produces:

```
  Classification  Random        X
0  SA EQUITY CFD       1  correct
1            bbb       2      bbb
2      SA EQUITY       3  correct
3         SA SSF       4  correct
4            aaa       5      aaa
```
