Before this change if --s3-no-head was in use rclone didn't check the
multipart upload ETag at all. However the ETag is returned in the
final POST request when completing the object.
This change uses that ETag from the final POST if --s3-no-head is in
use, otherwise it uses the ETag from a fresh HEAD request.
See: https://forum.rclone.org/t/in-some-cases-rclone-does-not-use-etag-to-verify-files/36095/
b3e0672535