Assignment broken for sliced multi-indices
Consider the following data frame:

```
>>> df = pd.DataFrame(1.0, index=pd.MultiIndex.from_product([[0, 1], [0, 1]]), columns=['a'])
>>> df
       a
0 0  1.0
  1  1.0
1 0  1.0
  1  1.0
```

Taking a slice of this and assigning it to itself should leave the data unchanged, but look what happens:

```
>>> df.loc[(0,), :] = df.loc[(0,), :]
>>> df
       a
0 0  NaN
  1  NaN
1 0  1.0
  1  1.0
```

I'm on pandas version 0.20.1
