Using groupby with nlargest drops the groupby column if there is only one element per group
Apologies if this has been fixed in 0.25.2, it's not in the repository yet.

#### Code Sample

```python
import pandas as pd

df = pd.DataFrame({'C1':['abc','def'],'C2':[2,1]})
print(df.groupby("C1").C2.nlargest(1).reset_index())

df = pd.DataFrame({'C1':['abc','abc','def','def'],'C2':[2,3,2,1]})
print(df.groupby("C1").C2.nlargest(1).reset_index())

```
#### Problem description

Result of the first part, the C1 column is gone and there is an index column:
```
   index  C2
0      0   2
1      1   1
```

Result of the second part, the C1 column is there and there is a level_1 column:
```
    C1  level_1  C2
0  abc        1   3
1  def        2   2
```


#### Expected Output

I'm expecting the first output to have the same format as the second, that is the groupby not dropping its groupby-column.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.79-1-MANJARO
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : de_DE.utf8
LOCALE           : de_DE.UTF-8

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.6.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
