Should SparseArray.astype be dense or Sparse

Right now `SparseArray.astype(numpy_dtype)` is sparse:

```python
In [6]: a = pd.SparseArray([0, 1, 0, 1])

In [7]: a.astype(np.dtype('float'))
Out[7]:
[0, 1.0, 0, 1.0]
Fill: 0
IntIndex
Indices: array([1, 3], dtype=int32)
```

This is potentially confusing. I did it to match the behavior of SparseSeries, but we may not want that.