SparseSeries.__array__ only returns non-fills
#### Code Sample, a copy-pastable example if possible

``` python
>>> np.array(pd.SparseSeries([0, 1], fill_value=0))
array([1])
```
#### Expected Output

```
array([0, 1])
```

this should really be consistent with `Series` rather than just returning the non-fill values (i.e. rather than being equivalent to `np.array(ps.SparseArray([np.nan, 1]))`.
#### output of `pd.show_versions()`

Pandas 0.18.1

should alone be relevant.

Apologies I've not checked if this is fixed in master. Just passing on issues from 
scikit-learn/scikit-learn#7352.
