ENH: SparseDataFrame/SparseSeries value assignment
- [x] closes https://github.com/pandas-dev/pandas/issues/21818
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Works by as much as possible using `SparseBlock.setitem()` which calls into `SparseArray.set_values()`, which returns a new (replacement) `SparseArray` object.