API: setting np.nan into Series[bool]
```
ser = pd.Series([True, False, True, False])
ser[0] = np.nan

>>> ser
0    NaN
1    0.0
2    1.0
3    0.0
dtype: float64
```

It would be more consistent (and would simplify the code) if this cast to object and did not convert True/False to 1.0/0.0.  ATM we have 2 tests that such a change would break.