GroupBy idxmin gives incorrect result on single-row data frame
#### Code Sample, a copy-pastable example if possible

```python
    def test_idxmin(self):
        df = pd.DataFrame({'accountid': ['12345'], 'createddate': [pd.Timestamp(2019, 1, 1)]})
        min = df.groupby('accountid')[['createddate']].idxmin(axis=1)
        print(min)

        df2 = pd.DataFrame({'accountid': ['12345', '23456'], 'createddate': [pd.Timestamp(2019, 1, 1), pd.Timestamp(2019, 2, 1)]})
        min2 = df2.groupby('accountid')[['createddate']].idxmin(axis=1)
        print(min2)

```
#### Problem description

The first print statement shows:
                     0
accountid             
12345      createddate

Current behavior appears to swap the column name and minimizer value.

The second print statement shows the correct result:
           createddate
accountid             
12345                0
23456                1

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

<details>

INSTALLED VERSIONS
------------------
commit           : None

pandas           : 0.25.1
numpy            : 1.15.2
pytz             : 2018.7
dateutil         : 2.7.5
pip              : 19.0.3
setuptools       : 40.5.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.7.5 (dt dec pq3 ext lo64)
jinja2           : 2.10
IPython          : 5.0.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : 0.3.0
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.0.1
numexpr          : None
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : 0.11.0
pytables         : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : 1.2.12
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
