BUG:Resample with groupby & agg()
- [ X] I have checked that this issue has not already been reported.

- [ all versions] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np

data = pd.DataFrame({
    'cat': ['cat_1', 'cat_1', 'cat_2', 'cat_1', 'cat_2', 'cat_1', 'cat_2', 'cat_1'],
    'num': [5,20,22,3,4,30,10,50],
    'date': ['2019-2-1', '2018-02-03','2020-3-11','2019-2-2', '2019-2-2', '2018-12-4','2020-3-11', '2020-12-12']
})
data['date'] = pd.to_datetime(data['date'])
aggreg = data.groupby('cat').resample('Y', on='date')
summ_ = aggreg.sum()
agg_summ_ = aggreg.agg({'num': 'sum'})
summ_
agg_summ_

```

#### Problem description

When I want aggregate all columns by sum, example with summ_ calculates normal, but if I do example with agg it calculates incorrect.

#### Expected Output
summ_
#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Sun Jul  5 00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : ru_RU.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0
numpy            : 1.18.4
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 20.0.2
setuptools       : 41.4.0
Cython           : 0.29.13
pytest           : 5.2.1
hypothesis       : None
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.1
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : 0.9.3
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : 1.2.1
fsspec           : 0.5.2
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.9
tables           : 3.5.2
tabulate         : 0.8.7
xarray           : 0.16.0
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.45.1

</details>
