ListObjectsV2 uses incorrect XML structure for `<Owner>` element

[korydraughn]

From https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListObjectsV2.html#API_ListObjectsV2_ResponseSyntax.

I've stripped the examples so only the important parts are visible. Notice the <Owner> elements.

<?xml version="1.0" encoding="UTF-8"?>
<ListBucketResult>
   <Contents>
      <Owner>
         <DisplayName>string</DisplayName>
         <ID>string</ID>
      </Owner>
   </Contents>
</ListBucketResult>

Here's what the S3 API returns.

<?xml version="1.0" encoding="utf-8"?>
<ListBucketResult>
    <Contents>
        <Owner>kory</Owner>
    </Contents>
</ListBucketResult>

[korydraughn]

Related to #225.

Fixing this issue requires determining which owner to return.

According to the AWS S3 docs, the <Owner> info is not included by default. Users must set the fetch-owner parameter to true for it to be included in the response. See #225 (comment).

Choosing to return the HTTP status code for Not Implemented when fetch-owner is true might negatively impact GUIs. That said, it's possible that the GUIs we tested do not care about the owner at all given the fact that the S3 API has been returning the wrong XML structure for <Owner> since its initial release.