### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Initiating a multipart S3 upload to LocalStack returns an XML document with a root element CreateMultipartUploadOutput. This causes an error in the ex_aws_s3 library I'm using. We've used the combination of LocalStack + ex_aws_s3 successfully for a few years and the error started occurring a few days ago.

### Expected Behavior

The element should be called InitiateMultipartUploadResult.

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run -p 4566:4566 localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    aws --debug --endpoint-url=http://localhost:4566 s3api create-multipart-upload --bucket my-bucket --key 'multipart/01'


### Environment

```markdown
- OS: Ubuntu 20.04.5 LTS
- LocalStack: latest
```


### Anything else?

S3 docs specifying InitiateMultipartUploadResult:
https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateMultipartUpload.html

Reference in official AWS Java client:
https://github.com/aws/aws-sdk-java/blob/ef22b94750d4c155d74b09012b3be5d9d98bbd33/aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/model/transform/XmlResponsesSaxParser.java#L1865

Where our particular error gets raised in ex_aws_s3:
https://github.com/ex-aws/ex_aws_s3/blob/7e12f2b0578b2620c047d2e44c40a6de8773584a/lib/ex_aws/s3/parsers.ex#L78
rators=(",", ":")))
        else:
            return value
    def _partition_members(self, parameters: dict, shape: Optional[Shape]) -> Tuple[dict, dict]:
        """Separates the top-level keys in the given parameters dict into header- and payload-located params."""
        if not isinstance(shape, StructureShape):
            # If the shape isn't a structure, we default to the whole response being parsed in the body.
            # Non-payload members are only loaded in the top-level hierarchy and those are always structures.
            return {}, parameters
        header_params = {}
        payload_params = {}
        shape_members = shape.members
        for name in shape_members:
            member_shape = shape_members[name]
            if name not in parameters:
                continue
            ")
            :
                header_params[name] = parameters[name]
            else:
                payload_params[name] = parameters[name]
        return header_params, payload_params
class RestXMLResponseSerializer(BaseRestResponseSerializer, BaseXMLResponseSerializer):
    """
    The ``RestXMLResponseSerializer`` is responsible for the serialization of responses from services with the
    ``rest-xml`` protocol.
    It combines the ``BaseRestResponseSerializer`` (for the ReST specific logic) with the ``BaseXMLResponseSerializer``
    (for the XML body response serialization).
    """
    pass
class QueryResponseSerializer(BaseXMLResponseSerializer):
    """
    The ``QueryResponseSerializer`` is responsible for the serialization of responses from services which use the
    ``query`` protocol. The responses of these services also use XML. It is basically a subset of the features, since it
    does not allow any payload or location traits.
    """
    def _serialize_r
        self,
        parameters: dict,
        response: HttpResponse,
        shape: Optional[Shape],
        shape_members: dict,
        operation_model: OperationModel,
        mime_type