NamedAgg with as_index= False drops aggregated values
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

file_name = "https://raw.githubusercontent.com/uiuc-cse/data-fa14/gh-pages/data/iris.csv"
df = pd.read_csv(file_name)

df[['species','petal_width']].groupby('species', as_index=False).agg(min_width=pd.NamedAgg(column='petal_width', aggfunc='min'))

```
#### Problem description
When I try to use `as_index=False`with named aggregations it seems to be dropping values and replacing them with index. This is the output of the above code.
|    | min_width   |
|---:|:------------|
|  0 | setosa      |
|  1 | versicolor  |
|  2 | virginica   |


#### Expected Output
|    | species    |   min_width |
|---:|:-----------|--------------:|
|  0 | setosa     |           0.1 |
|  1 | versicolor |           1   |
|  2 | virginica  |           1.4 |
#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.138+
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.16.4
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1
setuptools       : 41.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.5.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : 1.3.4
tables           : None
tabulate         : None
xarray           : 0.12.2
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
```
</details>
