style format setting subset loses column titles
See example below. There is no "sex" in the styled table.

```
from IPython.display import display as d
import pandas as pd
df=pd.DataFrame.from_dict(dict(survived=[1,0,0,0,1,1,1,1], sex=["Male", "Female", "Male", "Female", 
                                                                "Male", "Female","Female", "Female" ]))
table=pd.crosstab(df.survived, df.sex)
table.loc["rate"] = table.iloc[-1] / table.sum()
d(table)
ftable = table.style.format("{:.0%}", subset=pd.IndexSlice["rate",:])
d(ftable)
```
