BUG: bug when trying to display an embedded PandasObject (GH5324)

closes #5324

There was a bug where this wasn't display at all, so this PR fixes

Do we care about formatting *embedded* pandas objects? (as we don't recommend this anyhow)

```
In [15]: df1 = DataFrame({'a':[1, 2, 3], 'b':[3, 4, 5]})

In [19]: df2.dtypes
Out[19]: 
0    object
dtype: object

In [16]: df1
Out[16]: 
   a  b
0  1  3
1  2  4
2  3  5

In [17]: df2 = DataFrame([df1,df1+10])

In [18]: df2
Out[18]: 
                                         0
0             a  b
0  1  3
1  2  4
2  3  5
1      a   b
0  11  13
1  12  14
2  13  15
```