BUG: astype fill_value for SparseArray.astype
I don't think we have a specific issue for this. This is not a fix / change for https://github.com/pandas-dev/pandas/issues/23125

This fixes strange things like

```python
In [1]: import pandas as pd; import numpy as np

In [2]: a = pd.SparseArray([0, 1])

In [3]: a.astype(bool)
Out[3]:
[0, True]
Fill: 0
IntIndex
Indices: array([1], dtype=int32)
```

restoring the behavior of 0.23.x