Choose proper XML label for structured payload

[takumn]

Some XML payloads built for requesting REST API are not compatible with syntax.

Sample request from aws-cli:

aws s3api put-object-acl --bucket some-bucket-name --key some-key --access-control-policy '
{
    "Grants": [
        {
            "Grantee": {
                "ID": "3423e6294e24539a765a1430979cc6c619499eeb75ff49a134c851a87998ec28d569965bfbc868db0720ee92ce0cb01b",
                "Type": "CanonicalUser"
            }, 
            "Permission": "READ"
        }
    ],
    "Owner": {
        "ID": "1a4fa782449409d942e59e1082d3140d67df45d5eb4fffe68e7734519c30c202"
    }
}'

The following XML payload is expected for this request:
http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPUTacl.html#RESTObjectPUTacl-requests

<AccessControlPolicy>
  <AccessControlList>
    <Grant>
      <Grantee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
        <ID>3423e6294e24539a765a1430979cc6c619499eeb75ff49a134c851a87998ec28d569965bfbc868db0720ee92ce0cb01b</ID>
      </Grantee>
      <Permission>READ</Permission>
    </Grant>
  </AccessControlList>
  <Owner>
    <ID>1a4fa782449409d942e59e1082d3140d67df45d5eb4fffe68e7734519c30c202</ID>
  </Owner>
</AccessControlPolicy>

but given:

<AccessControlPolicy>
  <Grants>
    <AccessControlList>
      <Grantee xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser">
        <ID>3423e6294e24539a765a1430979cc6c619499eeb75ff49a134c851a87998ec28d569965bfbc868db0720ee92ce0cb01b</ID>
      </Grantee>
      <Permission>READ</Permission>
    </AccessControlList>
  </Grants>
  <Owner>
    <ID>1a4fa782449409d942e59e1082d3140d67df45d5eb4fffe68e7734519c30c202</ID>
  </Owner>
</AccessControlPolicy>

[jamesls]

Sorry for the delay. I've added a unit test for this and merged it via b1a2d62. Thanks!