BUG: bug in non-evently divisible offsets when resampling (e.g. '7s') (GH8371)

closes #8371

```
In [1]:         data = """date,time,value
   ...: 11-08-2014,00:00:01.093,1
   ...: 11-08-2014,00:00:02.159,1
   ...: 11-08-2014,00:00:02.667,1
   ...: 11-08-2014,00:00:03.175,1
   ...: 11-08-2014,00:00:07.058,1
   ...: 11-08-2014,00:00:07.362,1
   ...: 11-08-2014,00:00:08.324,1
   ...: 11-08-2014,00:00:08.830,1
   ...: 11-08-2014,00:00:08.982,1
   ...: 11-08-2014,00:00:09.815,1
   ...: 11-08-2014,00:00:10.540,1
   ...: 11-08-2014,00:00:11.061,1
   ...: 11-08-2014,00:00:11.617,1
   ...: 11-08-2014,00:00:13.607,1
   ...: 11-08-2014,00:00:14.535,1
   ...: 11-08-2014,00:00:15.525,1
   ...: 11-08-2014,00:00:17.960,1
   ...: 11-08-2014,00:00:20.674,1
   ...: 11-08-2014,00:00:21.191,1"""

In [2]:         df = pd.read_csv(StringIO(data), parse_dates={'timestamp': ['date', 'time']}, index_col='timestamp')

In [3]: df
Out[3]: 
                            value
timestamp                        
2014-11-08 00:00:01.093000      1
2014-11-08 00:00:02.159000      1
2014-11-08 00:00:02.667000      1
2014-11-08 00:00:03.175000      1
2014-11-08 00:00:07.058000      1
2014-11-08 00:00:07.362000      1
2014-11-08 00:00:08.324000      1
2014-11-08 00:00:08.830000      1
2014-11-08 00:00:08.982000      1
2014-11-08 00:00:09.815000      1
2014-11-08 00:00:10.540000      1
2014-11-08 00:00:11.061000      1
2014-11-08 00:00:11.617000      1
2014-11-08 00:00:13.607000      1
2014-11-08 00:00:14.535000      1
2014-11-08 00:00:15.525000      1
2014-11-08 00:00:17.960000      1
2014-11-08 00:00:20.674000      1
2014-11-08 00:00:21.191000      1

In [4]: df.resample('6s', how='sum')
Out[4]: 
                     value
timestamp                 
2014-11-08 00:00:00      4
2014-11-08 00:00:06      9
2014-11-08 00:00:12      4
2014-11-08 00:00:18      2
In [5]: df.resample('7s', how='sum')
Out[5]: 
                     value
timestamp                 
2014-11-08 00:00:00      4
2014-11-08 00:00:07     10
2014-11-08 00:00:14      4
2014-11-08 00:00:21      1

In [6]: df.resample('11s', how='sum')
Out[6]: 
                     value
timestamp                 
2014-11-08 00:00:00     11
2014-11-08 00:00:11      8

In [7]: df.resample('13s', how='sum')
Out[7]: 
                     value
timestamp                 
2014-11-08 00:00:00     13
2014-11-08 00:00:13      6

In [8]: df.resample('17s', how='sum')
Out[8]: 
                     value
timestamp                 
2014-11-08 00:00:00     16
2014-11-08 00:00:17      3
```