Copying of all headers for PUT request invalid in some cases

[jrnt30]

PR #39 has introduced an issue when using the x-amz-acl header. When using this header in conjunction with the mpu upload, the request will fail with a message of "The XML you provided was not well-formed or did not validate against our published schema"

I believe this is due to a limited subset of headers being valid for the PUT object request for the multipart upload.