BUG: (GH3925) partial string selection with seconds resolution

this no longer has much to do with #3925, and is only fixing a bug

Minor revision to select on second frequency
```
In [11]: df = DataFrame(randn(5,5),columns=['open','high','low','close','volume'],index=date_range('2012-01-02 18:01:00',periods=5,tz='US/Central',freq='s'))

In [12]: df
Out[12]: 
                               open      high       low     close    volume
2012-01-02 18:01:00-06:00  0.131243  0.301542  0.128027  0.804162  1.296658
2012-01-02 18:01:01-06:00  0.341487  1.548695  0.703234  0.904201  1.422337
2012-01-02 18:01:02-06:00 -1.050453 -1.884035  1.537788 -0.821058  0.558631
2012-01-02 18:01:03-06:00  0.846885  1.045378 -0.722903 -0.613625 -0.476531
2012-01-02 18:01:04-06:00  1.186823 -0.018299 -0.513886 -1.103269 -0.311907

In [14]: df['2012-01-02 18:01:02']
Out[14]: 
                               open      high       low     close    volume
2012-01-02 18:01:02-06:00 -1.050453 -1.884035  1.537788 -0.821058  0.558631

```