BUG: Panel4D setitem by indexer to Series fails for mixed-dtype

This is another tweak to `_setitem_with_indexer` that allows it better to comprehend higher dimensional frames of mixed-dtype. With this tweak setting by indexer to a Series now works. Unfortunately setting to a DataFrame is still broken. I plan to open an issue for that.
actually this should be ``self.obj[item].axes[0]``. The 0th axis. ``_stat_axis`` shouldn't work for a Panel4D because the Panel4D becomes a Panel (the ``self.obj[item]``), and ``_stat_axis`` is 1. So it may happen to work. Maybe add a couple of tests here.