BUG: Assigning Panel item using .ix/.loc no longer works
I am updating a codebase from an OLD version of pandas (0.7.3) to current version (0.14.1).

```
pandas: 0.14.1
numpy: 1.8.0
```

The following code has stopped working correctly (using .ix[] or .loc[]):

``` python
import numpy as np
import pandas as ps

np.random.seed(0)

index=range(3)
columns = list('abc')

panel = ps.Panel({'A' : ps.DataFrame(np.random.randn(3, 3), index=index, columns=columns),
                  'B' : ps.DataFrame(np.random.randn(3, 3), index=index, columns=columns),
                  'C' : ps.DataFrame(np.random.randn(3, 3), index=index, columns=columns)
                  })

replace = ps.DataFrame(np.eye(3,3), index=range(3), columns=columns)

print panel['A']

for idx in list('ABC'):
    panel.loc[idx,:,:] = replace

print panel['A']
```

Output:

```
          a         b         c
0  1.764052  0.400157  0.978738
1  2.240893  1.867558 -0.977278
2  0.950088 -0.151357 -0.103219
    a   b   c
0 NaN NaN NaN
1 NaN NaN NaN
2 NaN NaN NaN
```

However, if I simply do this:

``` python
for idx in list('ABC'):
    panel[idx] = replace

print panel['A']
```

It behaves as expected:

```
   a  b  c
0  1  0  0
1  0  1  0
2  0  0  1
```

Also, if I assign more than just the item, it works fine:

``` python
panel.loc['A', 0, 'a'] = 1.0
print panel['A']
```

```
          a         b         c
0  1.000000  0.400157  0.978738
1  2.240893  1.867558 -0.977278
2  0.950088 -0.151357 -0.103219
```

Thanks.
