API: SparseSeries comparison now returns sparse
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Because #13985 has been fixed, `SparseSeries` comparison op now can return `SparseSeries` (it returns normal `Series` on current master). 

also fixed a bug when `SparseArray` created from `SparseSeries` may not inherit `dtype`.
