BUG: resample in single group case not using how passed function
```
In [34]: s = Series([30.1,31.6],index=[Timestamp('20070915 15:30:00'),Timestamp('20070915 15:40:00')])

In [35]: s
Out[35]: 
2007-09-15 15:30:00    30.1
2007-09-15 15:40:00    31.6
dtype: float64

In [36]: s.resample('D',how=lambda x: np.std(x))
Out[36]: 
2007-09-15   NaN
Freq: D, dtype: float64

In [37]: s.groupby(pd.TimeGrouper('D')).apply(lambda x: np.std(x))
Out[37]: 
2007-09-15    0.75
dtype: float64

```
