series created with DatetimeIndex when index is [None]
This looks like a change from 0.13.1 to HEAD. If you create a series like this:

``` python
pd.Series(index=np.array([None]))
```

Then in 0.13.1 the result is:

``` python
Out[16]: 
NaN   NaN
dtype: float64

In [19]: x.index.dtype
Out[19]: dtype('O')

In [20]: type(x.index)
Out[20]: pandas.core.index.Index
```

but in HEAD the result is 

``` python
Out[191]: 
NaT   NaN
dtype: float64

In[192]: x.index.dtype
Out[192]: dtype('<M8[ns]')

In[193]: type(x.index)
Out[193]: pandas.tseries.index.DatetimeIndex
```

So the index has changed from Index/object to DatetimeIndex/datetime64.
