Fix request headers for multipart

[DrJosh9000]

Description

Set headers in the request that S3 is happy with.

Content-Range isn't necessary for the request (it was a flourish I added for debugging purposes), and Transfer-Encoding is added by Go when ContentLength isn't set.

Context

Testing showed that AWS rejects Content-Range and Transfer-Encoding headers with 501 Not Implemented.

Changes

• Remove Content-Range
• Set req.ContentLength
• Update the comment

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)