Before this change rclone used presigned requests to upload single
part objects. This was because of a limitation in the SDK which didn't
allow non seekable io.Readers to be passed in.
This is incompatible with some S3 backends, and rclone wasn't adding
the `X-Amz-Content-Sha256: UNSIGNED-PAYLOAD` header which was
incompatible with other S3 backends.
The SDK now allows for this so rclone can use PutObject directly.
This sets the `X-Amz-Content-Sha256: UNSIGNED-PAYLOAD` flag on the PUT
request. However rclone will add a `Content-Md5` header if at all
possible so the body data is still protected.
Note that the old behaviour can still be configured if required with
the `use_presigned_request` config parameter.
Fixes#5422
e5974ac4b0