BUG: assignment in Float64Index
http://stackoverflow.com/questions/24446500/pandas-valueerror-when-assigning-dataframe-entries-using-index-due-to-a-change

```
In [2]: A = pd.DataFrame(np.random.rand(10,4), index=np.random.rand(10))

In [3]: A.loc[A.index] = A.loc[A.index]
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```
