MinIO: InvalidCredsError when trying to retrieve an object

## Summary

When using the `minio` provider and getting an object, the method raises an `libcloud.common.types.InvalidCredsError`, despite the fact that it was able to create a container and upload it the object (so it doesn't look like a credential issue)
100% repro, code to repro the bug is provided

## Detailed Information

libcloud: latest stable, 3.3.1
Python: 3.8.10
OS: Ubuntu 20.04

Here you can find a repo to repro the bug, using `pytest` (Actually there are 2 bugs to report):
https://github.com/Wenzel/libcloud_bug

~~~python3
$ pytest -k test_demo_InvalidCreds
__________________________________________________________________________________________ test_demo_InvalidCreds ___________________________________________________________________________________________

clean_minio_db = , tmp_path = PosixPath('/tmp/pytest-of-wenzel/pytest-22/test_demo_InvalidCreds0')

def test_demo_InvalidCreds(clean_minio_db, tmp_path):
# setup
expected_data = b"data"
with open(tmp_path / "file1.txt", "wb") as f:
f.write(expected_data)
adapter = clean_minio_db
c = adapter.create_container("objects")
# test
ret = adapter.upload_object(tmp_path / "file1.txt", c, "file1")
> obj = adapter.get_object(c, "file1")

tests/test_demo_bug.py:31:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../.cache/pypoetry/virtualenvs/libcloud-bug-IT-EyGKG-py3.8/lib/python3.8/site-packages/libcloud/storage/drivers/s3.py:370: in get_object
response = self.connection.request(object_path, method='HEAD')
../../.cache/pypoetry/virtualenvs/libcloud-bug-IT-EyGKG-py3.8/lib/python3.8/site-packages/libcloud/common/base.py:655: in request
response = responseCls(**kwargs)
../../.cache/pypoetry/virtualenvs/libcloud-bug-IT-EyGKG-py3.8/lib/python3.8/site-packages/libcloud/common/base.py:166: in __init__
message=self.parse_error(),
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self =

def parse_error(self):
if self.status in [httplib.UNAUTHORIZED, httplib.FORBIDDEN]:
> raise InvalidCredsError(self.body)
E libcloud.common.types.InvalidCredsError: ''

../../.cache/pypoetry/virtualenvs/libcloud-bug-IT-EyGKG-py3.8/lib/python3.8/site-packages/libcloud/storage/drivers/s3.py:138: InvalidCredsError
------------------------------------------------------------------------------------------- Captured stdout setup -------------------------------------------------------------------------------------------
ca19a7e29a92d875ec58b0285c854403ae5a95433a71e084011d32018a9141d1
----------------------------------------------------------------------------------------- Captured stdout teardown ------------------------------------------------------------------------------------------
libcloud_bug_objectmistmatch_miniodb
========================================================================================== short test summary info ==========================================================================================
FAILED tests/test_demo_bug.py::test_demo_InvalidCreds - libcloud.common.types.InvalidCredsError: ''
~~~

Thanks for your help !

tests/test_demo_bug.py から始め、pytest -k test_demo_InvalidCreds を使って失敗を再現します。s3.py 内の MinIO get_object を、特に S3Response.parse_error と HEAD リクエストを中心に追跡し、その後、アップロードしたオブジェクトの取得で InvalidCredsError が発生しないことを示す回帰テストのカバレッジを追加します。同時に、本物の認証情報エラーは引き続き識別できるようにします。

索引モデルが issue の本文から書いたものです。