Discretizing a group of float columns using (apply + cut ) does not work
This does not work

``` python
df = pd.DataFrame(np.arange(20).reshape(4,5)).astype(float)
df.apply(lambda x:pd.cut(x,2))
```

Here the result is pd.Series

This  works

``` python
df = pd.DataFrame(np.arange(20).reshape(4,5))
df.apply(lambda x:pd.cut(x,2))
```

Here the result is a dataframe
