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.
Agreed. This is turning out to be a lot less trivial than I originally thought.