BUG: ufunc is not applied to sparse.fill_value
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

When ufunc is applied to sparse, it is not applied to `fill_value`. Thus results are incorrect.

**on current master:**

```
np.abs(pd.SparseArray([1, -2, -1], fill_value=-2))
# [1.0, -2, 1.0]
# Fill: -2
# IntIndex
# Indices: array([0, 2], dtype=int32)

np.add(pd.SparseArray([1, -2, -1], fill_value=-2), 1)
# [2.0, -2, 0.0]
# Fill: -2
# IntIndex
# Indices: array([0, 2], dtype=int32)
```

cc @gfyoung 
