### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

There is an issue with `get-object-attributes` command which results in below error always: 
`aws --endpoint-url=http://localhost:4566 s3api get-object-attributes --bucket s3mock --key 20.txt --object-attributes "ObjectSize"`
````

> Response:  Unable to parse response (syntax error: line 1, column 0), invalid XML received. Further retries may succeed:
b'98\n5\n'
````

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### Using docker-compse.yaml to start localstack

docker-compse.yaml:

```
localstack:
    image: localstack/localstack:latest
    ports:
      - '4563-4599:4563-4599'
      - '8055:8080'
    environment:
      - SERVICES=s3
      - DEBUG=1
      - LS_LOG=error
      - DATA_DIR=/tmp/localstack/data
      - AWS_DEFAULT_REGION=eu-west-1
      # - EDGE_PORT=4566
    volumes:
      - "./.localstack:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
```

#### cli commands to reproduce the issue

1. Create a Bucket: `aws --endpoint-url=http://localhost:4566 s3 mb s3://s3mock`
2. Create an Object: `aws --endpoint-url=http://localhost:4566 s3 cp data/20.txt s3://s3mock`

File 20.txt content are 2-digit numbers separated by newline.

3. get-object-attributes for object 20.txt :
```

 aws --endpoint-url=http://localhost:4566 s3api get-object-attributes --bucket s3mock --key 20.txt --object-attributes "ObjectSize"

```
Results in error:

> Unable to parse response (syntax error: line 1, column 0), invalid XML received. Further retries may succeed:
b'98\n5\n'



### Environment

```markdown
- OS:Mac 12.x
- LocalStack:latest
- Node 14
```


### Anything else?

_No response_
"Attribute.1.Name=DelaySeconds&"
            "Attribute.1.Value=10&"
            "Attribute.2.Name=MaximumMessageSize&"
            "Attribute.2.Value=131072&"
            "Attribute.3.Name=MessageRetentionPeriod&"
            "Attribute.3.Value=259200&"
            "Attribute.4.Name=ReceiveMessageWaitTimeSeconds&"
            "Attribute.4.Value=20&"
            "Attribute.5.Name=RedrivePolicy&"
            "Attribute.5.Value=%7B%22deadLetterTargetArn%22%3A%22arn%3Aaws%3Asqs%3Aus-east-1%3A80398EXAMPLE%3AMyDeadLetterQueue%22%2C%22maxReceiveCount%22%3A%221000%22%7D&"
            "Attribute.6.Name=VisibilityTimeout&Attribute.6.Value=60"
        ),
        method="POST",
        headers={"Content-Type": "application/x-www-form-urlencoded"},
        path="",
    )
    operation, params = parser.parse(request)
    assert operation.name == "SetQueueAttributes"
    assert params == {
        "QueueUrl": "http://localhost:4566/000000000000/tf-acc-test-queue",
        "Attributes": {
            "DelaySeconds": "10",
            "MaximumMessageSize": "131072",
            "MessageRetentionPeriod": "259200",
            "ReceiveMessageWaitTimeSeconds": "20",
            "RedrivePolicy": '{"deadLetterTargetArn":"arn:aws:sqs:us-east-1:80398EXAMPLE:MyDeadLetterQueue","maxReceiveCount":"1000"}',
            "VisibilityTimeout": "60",
        },
    }
def test_query_parser_non_flattened_map():