Setting values with multiindex .loc 
#### Code Sample

```python
df = pd.DataFrame({'A':[1,2,3],'B':[4,5,6]})
df = df.stack()
df.loc[0] = [0,0]
```
This returns 
```
In [27]: df
Out[27]:
0  A    0
   B    0
1  A    2
   B    5
2  A    3
   B    6
dtype: int64
```
Then, if I try 
```
df.loc[0] = df.loc[1]
```
I get
```
In [29]: df
Out[29]:
0  A    NaN
   B    NaN
1  A    2.0
   B    5.0
2  A    3.0
   B    6.0
dtype: float64
```
#### Expected Output
It should either raise an exception/warning, or produce the same result with `df.loc[0] = df.loc[1].values`. 

