Some S3 compatible object storage systems like R2 require that all
multipart chunks are the same size. This was mostly true before, except
the final chunk was larger than the requested chunk size which causes
uploads to fail.
In addition, the two byte slices have been replaced with a single
*bytes.Buffer and the surrounding code simplified significantly.
Fixes: #3873
Signed-off-by: Thomas Way <thomas@6f.io>
