Fix signature mismatch in putobject in python3 (Fixes #435) (#436)

Author: donatello

minio/signer.py

@@ -199,6 +199,8 @@ def sign_v4(method, url, region, headers=None, access_key=None,
     if content_sha256 is None:
         # with no payload, calculate sha256 for 0 length data.
         content_sha256 = encode_to_hex(get_sha256(b'')).decode('ascii')
+    elif isinstance(content_sha256, bytes):
+        content_sha256 = content_sha256.decode('ascii')
 
     host = parsed_url.netloc
     headers['Host'] = host