Resample is bugging in timeseries for certain start date indexes [edited]
Hi guys,

For certain values of start dates, the method `resample` seems to bug.

I explain with an example :

```
rng = pd.date_range('20110105 12:20:00',periods=5*60,freq='min')
tsMin=pd.Series(range(len(rng)), index=rng)
tsMin
2011-01-05 12:20:00     0
2011-01-05 12:21:00     1
2011-01-05 12:22:00     2
2011-01-05 12:23:00     3
...
2011-01-05 17:18:00    298
2011-01-05 17:19:00    299
Freq: T, Length: 300
```

When trying to resample in `300 minutes` it comes:

```
tsMin.resample("300min")
2011-01-05 10:00:00     79.5
2011-01-05 15:00:00    229.5
Freq: 300T, dtype: float64
```

The result is not what expected...

What is wired is that it works when I replace `2011-01-05 12:20:00` by `2011-01-05 00:00:00`

Thank you for your help
