Panel.shift does not respect dtypes 13.1
Because the 13.1 shift uses `.values`, it will upcast mixed dtypes to object. 

``` python
import pandas as pd
import numpy as np
import pandas.util.testing as tm


data = [('item '+ch, tm.makeMixedDataFrame()) for ch in list('abcde')]
data = dict(data)
mixed_panel = pd.Panel.from_dict(data, orient='minor')
shifted = mixed_panel.shift(1)
tm.assert_series_equal(mixed_panel.dtypes, shifted.dtypes) #fails
```

Have PR for this.
