Asymmetry between groupby([groupers]) and groupby(grouper).resample
Should these be the same?

```python
In [1]: import pandas as pd; import numpy as np

In [2]: paste
        df = pd.DataFrame({
            'A': [1] * 6 + [2] * 6,
            'B': pd.to_datetime(['2017'] * 3 +
                                ['2019'] * 3 +
                                ['2017'] * 3 +
                                ['2019'] * 3),
            'C': [1] * 12
        })
        a = df.groupby([pd.Grouper(key='B', freq='AS'), 'A']).C.count()
        b = (df.set_index("B").sort_index().groupby("A").resample("AS")
               .C.count())

## -- End pasted text --

In [3]: a
Out[3]:
B           A
2017-01-01  1    3
            2    3
2019-01-01  1    3
            2    3
Name: C, dtype: int64

In [4]: b
Out[4]:
A  B
1  2017-01-01    3
   2018-01-01    0
   2019-01-01    3
2  2017-01-01    3
   2018-01-01    0
   2019-01-01    3
Name: C, dtype: int64
```

xref https://github.com/pandas-dev/pandas/pull/18921#issuecomment-354179920l