ENH/BUG: broadcast a lower-dim assignment if possible
http://stackoverflow.com/questions/19346033/how-to-collectively-set-the-values-of-multiple-columns-for-certain-selected-rows/19346925#19346925

This does not work as we are using a rhs of a series which is not broadcast
to the assignment

```
df.loc[df['A'],['A','B']] = df['C']
```

This works, but a tad cumbersome

```
In [17]: df = DataFrame(dict(A = [1,2,0,0,0],B=[0,0,0,10,11],C=[3,4,5,6,7]))

In [18]: df
Out[18]: 
   A   B  C
0  1   0  3
1  2   0  4
2  0   0  5
3  0  10  6
4  0  11  7

In [19]: mask = df['A'] == 0

In [20]: for col in ['A','B']:
   ....:     df.loc[mask,col] = df['C']
   ....:     

In [21]: df
Out[21]: 
   A  B  C
0  1  0  3
1  2  0  4
2  5  5  5
3  6  6  6
4  7  7  7
```
