DataFrame with Int64 columns casts to float64 with .max()/.min()
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np

int64_info = np.iinfo("int64")
s = pd.Series([int64_info.max, None, int64_info.min], dtype=pd.Int64Dtype())
df = pd.DataFrame({"Int64": s})

df.max()
Int64    9.223372e+18
dtype: float64
```
#### Problem description

`pd.Int64` data is converted to `np.float64` in certain reduction operations on `pd.DataFrame`. This causes data corruption, as `pd.Int64` is intended to avoid this exact issue.

#### Expected Output
`df.max()` should probably return a `pd.Series` of `dtype='object'` wrapping a `pd.Int64` value.

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

<details>
```
INSTALLED VERSIONS
------------------
commit           : 27ad779713f1d54b83547db5465bec4245d279cd
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-29-generic
Version          : #31-Ubuntu SMP Fri Jan 17 17:27:26 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0.dev0+779.g27ad77971
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : 0.29.14
pytest           : 5.3.5
hypothesis       : 5.4.1
sphinx           : 2.4.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.4.0.dev0+62.g8ac3a4c8
fastparquet      : 0.3.2
gcsfs            : None
matplotlib       : None
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.11.1
pytables         : None
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : 0.14.1
xlrd             : None
xlwt             : None
numba            : 0.48.0
```
</details>