Fix sign v4 breakage in Python 3

[donatello]

It was introduced in commit id 31e8a35 - once this was found (with git bisect), the fix was easy to figure out.

_get_bucket_region() was failing in Python 3 with signature mismatch
error.

Short program that demonstrates the issue:

from minio import Minio
from minio.error import ResponseError
from sys import stdout
s3Client = Minio(
    'localhost:9000',
    access_key='minio',
    secret_key='minio123',
    secure=False,
)

s3Client.trace_on(stdout)

objects = s3Client.list_objects('aditya', recursive=True)

for c, obj in enumerate(objects):
    print(obj)
    if c > 10:
        break

Generates:

Traceback (most recent call last):
  File "../minio-test.py", line 21, in <module>
    for c, obj in enumerate(objects):
  File "/home/aditya/Code/minio-code/python/venv/lib/python3.5/site-packages/minio-2.0.3-py3.5.egg/minio/api.py", line 912, in list_objects
  File "/home/aditya/Code/minio-code/python/venv/lib/python3.5/site-packages/minio-2.0.3-py3.5.egg/minio/api.py", line 1699, in _url_open
  File "/home/aditya/Code/minio-code/python/venv/lib/python3.5/site-packages/minio-2.0.3-py3.5.egg/minio/api.py", line 1645, in _get_bucket_region
  File "/home/aditya/Code/minio-code/python/venv/lib/python3.5/site-packages/minio-2.0.3-py3.5.egg/minio/api.py", line 1678, in _get_bucket_location
minio.error.ResponseError: ResponseError: code: SignatureDoesNotMatch, message: The request signature we calculated does not match the signature you provided. Check your key and signing method., bucket_name: None, object_name: None, request_id: RUSVQ0114N14O0ZG, host_id: 3L137, region: 

[krisis]

LGTM

[harshavardhana]

content_sha256 can be "UNSIGNED_PAYLOAD" ".decode will fail in that scenario. Do it like this

if content_sha256 == _UNSIGNED_PAYLOAD: 
     headers['X-Amz-Content-Sha256'] = content_sha256
else:
     headers['X-Amz-Content-Sha256'] = content_sha256.decode('ascii')

[vadmeste]

👍

[donatello]

It seems pretty clumsy to keep content_sha256 as bytes in one case and as str in the other. I am making a slightly better change to handle this case.

[harshavardhana]

It is but it should be handled in a way UNSIGNED_PAYLOAD should still be a string, keeping it as byte causes presigned signature requests to fail.

[harshavardhana]

same as above.