When uploading or moving data within an s3-compatible bucket, the
`SSECustomer*` headers should always be forwarded: on
`CreateMultipartUpload`, `UploadPart`, `UploadCopyPart` and
`CompleteMultipartUpload`. But currently rclone doesn't forward those
headers to `CompleteMultipartUpload`.
This is a requirement if you want to enforce `SSE-C` at the bucket level
via a bucket policy. Cf: `This parameter is required only when the
object was created using a checksum algorithm or if your bucket policy
requires the use of SSE-C.` in
https://docs.aws.amazon.com/AmazonS3/latest/API/API_CompleteMultipartUpload.html
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