Use repr of fill_value in SparseDtype repr
This proposes changing the StringDtype repr to show the fill_value's repr rather than its string representation:

```python
>>> arr = pd.arrays.SparseArray([0, 1])
>>> arr.dtype
Sparse[int64, 0]  # master & this PR, unchanged
>>> arr.astype(str).dtype
Sparse[object, 0]  # master
Sparse[object, '0']  # this PR, notice quotes
```
