Groupby grouper by datetime broken?
Behaviour:

```
start, end = '2000-10-01 23:30:00', '2000-10-02 00:30:00'

rng = pd.date_range(start, end, freq='7min')

ts = pd.DataFrame(data = {'timestamp': rng, 'col2': ['cat', 'dog','cat', 'dog','cat', 'dog','cat', 'dog','cat']})

ts.groupby(['col2', pd.Grouper(key="timestamp", freq='60min', `origin='start')]).cumcount()

ts
``` 

0    0
1    0
2    1
3    1
4    2
5    0
6    0
7    1
8    1

Expected behaviour:

0    0
1    0
2    1
3    1
4    2
5    2
6    3
7    3
8    4