DOC: Update is_sparse docstring
`is_sparse(pd.SparseDataFrame([1,2,3]))` returns `False`.

The is_sparse function checks to see if
an array-like is spare by checking to see
if it is an instance of ABCSparseArray or
ABCSparseSeries. This commit adds
ABCSparseDataFrame to that list -- so it
can detect that a DataFrame (which is an
array-like object) is sparse.

Added a test for this.

- [ ] closes #xxxx
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
