BUG: resample apply is actually aggregate
- [x] I have checked that this issue has not already been reported.

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

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

---

this piece of code does it, I don't really understand why these things are OK to be the same function here
https://github.com/pandas-dev/pandas/blob/7ffbf1a5ab6b0126c7efa5991e7db505979a752c/pandas/core/resample.py#L308

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

```python
df = pd.DataFrame([[1,2],[3,4], [5,6]], index=pd.date_range("2020-01-30", "2020-02-01"))

df.resample("1M").apply(lambda df: df.mean().mean())

```

#### Problem description

current behavior calls the function on all columns, in all groups, as in aggregate, but is should on all dfs of the groups as in apply

#### Expected Output

```
2020-01-31    2.5
2020-02-29    5.5
Freq: M, dtype: float64
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.2.1-1.el7.elrepo.x86_64
Version          : #1 SMP Sun Jul 14 08:15:04 EDT 2019
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.2
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.1.0
Cython           : None
pytest           : 4.6.11
hypothesis       : 5.29.0
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.5 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.17.0
pandas_datareader: None
bs4              : None
bottleneck       : 1.3.2
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.19
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
