ts.resample quietly does nothing when resampling with how="ffill"
``` python
ix = pd.date_range('1/1/2013', periods=12, freq='M')
ts = Series(np.arange(12), index=ix)
ts
2013-01-31     0
2013-02-28     1
2013-03-31     2
2013-04-30     3
2013-05-31     4
2013-06-30     5
2013-07-31     6
2013-08-31     7
2013-09-30     8
2013-10-31     9
2013-11-30    10
2013-12-31    11
Freq: M, dtype: int64

ts.resample('1D', how='ffill')
2013-01-31     0
2013-02-28     1
2013-03-31     2
2013-04-30     3
2013-05-31     4
2013-06-30     5
2013-07-31     6
2013-08-31     7
2013-09-30     8
2013-10-31     9
2013-11-30    10
2013-12-31    11
Freq: M, dtype: int64
```
