TST: groupby of idxmax/min with mixed types
- [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.

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

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**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
df.groupby('teamId').idxmax()
tmp.groupby('teamId').idxmax()
```


#### Problem description

DataFrame df is consist of **teamId(int64)** and **neutralMinionsKilled(int64)** and 'tmp' is consist of **teamId(Int64)** and **neutralMinionsKilled(Int64)**
`df.groupby('teamId').idxmax()` is working
But `tmp.groupby('teamId').idxmax()` is not working and showing 'TypeError: argmax() takes 1 positional argument but 2 were given' 

#### Expected Output
![image](https://user-images.githubusercontent.com/50440358/110598649-559fab80-81c5-11eb-8327-33928f36605c.png)


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

<details>

[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]
![image](https://user-images.githubusercontent.com/50440358/110598822-8c75c180-81c5-11eb-902a-1d743f0ad890.png)


</details>
