assignment in mixed-type dataframes not working?
```
from pandas import DataFrame
df=DataFrame({'a':{1:'aaa',2:'bbb',3:'ccc'},'b':{1:111,2:222,3:333}})
#this works, new column is created correctly
df['test']=df['a'].apply(lambda x: '_' if x=='aaa' else x)
#this does not work, ie column test is not changed
idx=df['test']=='_'
temp=df.ix[idx,'a'].apply(lambda x: '-----' if x=='aaa' else x)
df.ix[idx,'test']=temp
#this works, column b is changed
temp=df.ix[idx,'a'].apply(lambda x: 99999 if x=='aaa' else x)
df.ix[idx,'b']=temp
#this does not work
temp=df.ix[idx,'a'].apply(lambda x: 'zzzzzz' if x=='aaa' else x)
df.ix[idx,'a']=temp
```

code works correctly on 0.9, not on 0.11
