BUG: consistent datetime display format with < ms #10170
closes #10170

```
In [3]: Series(date_range('20130101 09:00:00',periods=5,freq='D'))
Out[3]: 
0   2013-01-01 09:00:00
1   2013-01-02 09:00:00
2   2013-01-03 09:00:00
3   2013-01-04 09:00:00
4   2013-01-05 09:00:00
dtype: datetime64[ns]

In [4]: Series(date_range('20130101 09:00:00',periods=5,freq='s'))
Out[4]: 
0   2013-01-01 09:00:00
1   2013-01-01 09:00:01
2   2013-01-01 09:00:02
3   2013-01-01 09:00:03
4   2013-01-01 09:00:04
dtype: datetime64[ns]

In [2]: s = Series(date_range('20130101 09:00:00',periods=5,freq='ms'))

In [3]: s.iloc[1] = np.nan

In [4]: s
Out[4]: 
0   2013-01-01 09:00:00.000
1                       NaT
2   2013-01-01 09:00:00.002
3   2013-01-01 09:00:00.003
4   2013-01-01 09:00:00.004
dtype: datetime64[ns]

In [6]: Series(date_range('20130101 09:00:00',periods=5,freq='us'))
Out[6]: 
0   2013-01-01 09:00:00.000000
1   2013-01-01 09:00:00.000001
2   2013-01-01 09:00:00.000002
3   2013-01-01 09:00:00.000003
4   2013-01-01 09:00:00.000004
dtype: datetime64[ns]

In [7]: Series(date_range('20130101 09:00:00',periods=5,freq='N'))
Out[7]: 
0   2013-01-01 09:00:00.000000000
1   2013-01-01 09:00:00.000000001
2   2013-01-01 09:00:00.000000002
3   2013-01-01 09:00:00.000000003
4   2013-01-01 09:00:00.000000004
dtype: datetime64[ns]
```
