BUG: fancy setitem on a mixed-frame with a series fails
```
In [35]: df = pandas.DataFrame({'x':range(10), 'y':range(10,20),'z' : 'bar'})

In [36]: df.ix[df.x % 2 == 0, 'y'] = df.ix[df.x % 2 == 0, 'y'].values * 100

In [37]: df
Out[37]: 
   x     y    z
0  0  1000  bar
1  1    11  bar
2  2  1000  bar
3  3    13  bar
4  4  1000  bar
5  5    15  bar
6  6  1000  bar
7  7    17  bar
8  8  1000  bar
9  9    19  bar
```
