Aggregation does not preserve dtype when function passed in dictionary

[tsoernes]

Code Sample, a copy-pastable example if possible

In [92]: df_ = pd.DataFrame([[0, 1], [0, np.nan]]).convert_dtypes()

In [93]: df_
Out[93]: 
   0     1
0  0     1
1  0  <NA>

In [108]: df_.groupby(0).agg('last')
Out[111]: 
   1
0   
0  1

In [112]: df_.groupby(0).agg({1:'last'})
Out[115]: 
     1
0     
0  1.0


In [116]: df_.groupby(0).agg('last').dtypes
Out[116]: 
1    Int64
dtype: object

In [117]: df_.groupby(0).agg({1:'last'}).dtypes
Out[117]: 
1    float64
dtype: object

Problem description

Aggregation does not preserve Int64 dtype when function passed in dictionary for the column.

Expected Output

Int64 dtype

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.5.final.0
python-bits : 64
OS : Linux
OS-release : 5.5.8-100.fc30.x86_64
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : nb_NO.UTF-8
LOCALE : nb_NO.UTF-8

pandas : 1.0.2
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 19.3.1
setuptools : 41.6.0.post20191030
Cython : 0.29.13
pytest : 5.2.2
hypothesis : None
sphinx : 2.2.1
blosc : None
feather : None
xlsxwriter : 1.2.2
lxml.etree : 4.4.1
html5lib : 1.0.1
pymysql : None
psycopg2 : 2.8.4 (dt dec pq3 ext lo64)
jinja2 : 2.10.3
IPython : 7.9.0
pandas_datareader: None
bs4 : 4.8.1
bottleneck : 1.2.1
fastparquet : None
gcsfs : None
lxml.etree : 4.4.1
matplotlib : 2.2.3
numexpr : 2.7.0
odfpy : None
openpyxl : 3.0.0
pandas_gbq : None
pyarrow : 0.16.0
pytables : None
pytest : 5.2.2
pyxlsb : None
s3fs : None
scipy : 1.3.1
sqlalchemy : 1.3.10
tables : 3.5.2
tabulate : 0.8.5
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.2
numba : 0.46.0

[mroeschke]

Looks to work on master now. Could use a test

In [1]: In [92]: df_ = pd.DataFrame([[0, 1], [0, np.nan]]).convert_dtypes()
   ...:
   ...: In [93]: df_
Out[1]:
   0     1
0  0     1
1  0  <NA>

In [2]: In [108]: df_.groupby(0).agg('last')
Out[2]:
   1
0
0  1

In [3]: In [112]: df_.groupby(0).agg({1:'last'})
Out[3]:
   1
0
0  1

In [4]: In [116]: df_.groupby(0).agg('last').dtypes
Out[4]:
1    Int64
dtype: object

In [5]: In [117]: df_.groupby(0).agg({1:'last'}).dtypes
Out[5]:
1    Int64
dtype: object