setting values in panel using dataframe mask creates different results when selecting
apologies if this is a manifest of another issue, though I couldn't find one after some searching.

the following behavior can be seen in 0.14.1 and 0.15.2:

```
pt = pd.Panel({'a': {'b': {'c': 1, 'd': 1}, 'e': {'c': 2, 'd': 2}}})
pt2 = pd.Panel({'a': {'b': {'c': 3, 'd': 5}, 'e': {'c': 4, 'd': 6}}})
pt.a, pt2.a
mask = (pt.a > 1) & (pt2.a > 4)
pt.a[mask] = pt2.a[mask]
pt.loc[['a'],:,'e']
pt.loc['a',:,'e']
```

the pt.loc[['a'],:,'e'] shows the original 2 while pt.loc['a',:,'e'] shows the updated 6 from pt2.a

is there a better way to do this, basically i'd like to set the values of pt.a to that of pt2.a based on some mask.  any direction would be much appreciated, thanks!
