NamedAgg error: aggregate() missing 1 required positional argument: 'func'
#### Code Sample, a copy-pastable example if possible

```python
resampled_bookings = bookings.resample('BMS').agg(
    unique_identity_ids=pd.NamedAgg(column="identity_id", aggfunc=pd.Series.nunique),
    booking_count=pd.NamedAgg(column="booking_id", aggfunc=pd.Series.count),
    booking_distance=pd.NamedAgg(column="distance_m", aggfunc=pd.Series.sum)
)
```
#### Problem description

This error prevents me from aggregating data and (re)naming columns at the same time. Instead, it is necessary to use the following approach, which isn't altogether bad:

```py
resampled_bookings = bookings.resample('BMS').agg({
    "booking_id": "count",
    "distance_m": sum,
    "identity_id": pd.Series.nunique
})

resampled_bookings.rename(columns={
    "booking_id": "booking_count",
    "distance_m": "booking_distance_m",
    "identity_id": "unique_identity_ids"
}, inplace=True)
```

#### Expected Output
The aggregate function would produce a new DataFrame with the following columns:
- booking_count
- booking_distance_m
- unique_identity_ids

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-19-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.0
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
