BUG: support "fill_value" for ".unstack()" called with list of levels
- [x] closes #13971
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Notice this is a simple fix which doesn't make @kordek 's [more general comment](https://github.com/pandas-dev/pandas/issues/13971#issuecomment-243558072) about the opportunity for refactoring less valid.