ERR: invalid input to .str.replace does not raise
This should raise; `None` is not an allowed option for the replacement.

```
In [2]: s = pd.Series(['a', 'b', None])

In [3]: s
Out[3]: 
0       a
1       b
2    None
dtype: object

In [4]: s.str.replace('a', None)
Out[4]: 
0   NaN
1   NaN
2   NaN
dtype: float64
```

cc @kwsmith
