BUG: pivot_table sometimes returns Series (#4386)
- [x] closes #4386
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

The [`pivot_table` docs](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.pivot_table.html) mentions that a `DataFrame` should be returned. In some cases, a `Series` is returned. I had to also alter some existing tests that expected `pivot_table` to return a `Series`

Any feedback welcomed, thanks!
