### 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_
"""Simple test with a flattened map (SQS SetQueueAttributes request)."""
    parser = QueryRequestParser(load_service("sns"))
    request = HttpRequest(
        body=to_bytes(
            "Action=SetEndpointAttributes&"
            "EndpointArn=arn%3Aaws%3Asns%3Aus-west-2%3A123456789012%3Aendpoint%2FGCM%2Fgcmpushapp%2F5e3e9847-3183-3f18-a7e8-671c3a57d4b3&"
            "Attributes.entry.1.key=CustomUserData&"
            "Attributes.entry.1.value=My+custom+userdata&"
            "Version=2010-03-31&"
            "AUTHPARAMS"
        ),
        method="POST",
        headers={"Content-Type": "application/x-www-form-urlencoded"},
        path="",
    )
    operation, params = parser.parse(request)
    assert operation.name == "SetEndpointAttributes"
    assert params == {
        "Attributes": {"CustomUserData": "My custom userdata"},
        "EndpointArn": "arn:aws:sns:us-west-2:123456789012:endpoint/GCM/gcmpushapp/5e3e9847-3183-3f18-a7e8-671c3a57d4b3",
    }
def test_query_parser_non_flattened_list_structure():
    """Simple test with a non-flattened list structure (CloudFormation CreateChangeSet)."""
    parser = QueryRequestParser(load_service("cloudformation"))
    request = HttpRequest(
        body=to_bytes(
            "Action=CreateChangeSet&"