Series / DataFrame constructors inconsistent with data=None and dtype
The series constructed below should (I think) be a special case of the DataFrame example, but they differ.

```python
In [4]: pd.DataFrame(None, index=[1, 2, 3], columns=['a'], dtype=int)
Out[4]:
    a
1 NaN
2 NaN
3 NaN

In [5]: pd.Series(None, index=[1, 2, 3], dtype=int)
Out[5]:
1    0
2    0
3    0
dtype: int64
```

I don't know which makes more sense.