BUG: pandas not connecting to private S3 endpoint

#### Problem description

Connecting to  https://bucket.s3.amazonaws.com rather then private endpoint

```python
df = pd.read_parquet('s3://bucket/file.parquet', storage_options={'client_kwargs':{'endpoint_url': S3_ENDPOINT}})
```
as workaround I do:

```python
# ecs is a S3FileSystem object with client_kwargs

with ecs.open('bucket/file.parquet', 'rb') as f:
    df = pd.read_parquet(f)
```

#### Expected Output

Connecting to private S3 bucket

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-1127.19.1.el7.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.18.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 49.1.0.post20200704
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : 0.4.0
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.17.1
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : None
sqlalchemy       : 1.3.18
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.50.1

</details>
