BUG: setitem fails on mixed-type Panel4D

Trying to set a value on a mixed-type Panel4D currently fails:

    In [1]: from pandas.util import testing as tmd

    In [2]: p4d = tmd.makePanel4D()

    In [3]: p4d['foo'] = 'bar'

    In [4]: p4d.iloc[0,0,0,0] = 0
    ---------------------------------------------------------------------------
    AttributeError                            Traceback (most recent call last)
    <ipython-input-4-bb74b95f7244> in <module>()
    ----> 1 p4d.iloc[0,0,0,0] = 0

    /home/stahlous/pd_dev/pandas/pandas/core/indexing.py in __setitem__(self, key, value)
        119                 indexer = self._convert_to_indexer(key, is_setter=True)
        120
    --> 121         self._setitem_with_indexer(indexer, value)
        122
        123     def _has_valid_type(self, k, axis):

    /home/stahlous/pd_dev/pandas/pandas/core/indexing.py in _setitem_with_indexer(self, indexer, value)
        356             # indexer here
        357             if (len(labels) == 1 and
    --> 358                     isinstance(self.obj[labels[0]].index, MultiIndex)):
        359                 item = labels[0]
        360                 obj = self.obj[item]

    /home/stahlous/pd_dev/pandas/pandas/core/generic.py in __getattr__(self, name)
       1927         """
       1928         if name in self._internal_names_set:
    -> 1929             return object.__getattribute__(self, name)
       1930         elif name in self._metadata:
       1931             return object.__getattribute__(self, name)

    AttributeError: 'Panel' object has no attribute 'index'

Fortunately, it looks like a simple fix. If this looks OK. I'll add a note to `v0.15.1.txt` (doesn't have a GH issue number yet).

This fix is needed for my latest attempt at the fillna bug #8395.