### 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
ntent_type(
        self, serialized: HttpResponse, shape: Shape, shape_members: dict, mime_type: str
    ):
        """Set Content-Type to application/json for all structured bodies."""
        payload = shape.serialization.get("payload") if shape is not None else None
        if self._has_streaming_payload(payload, shape_members):
            # Don't apply content-type to streaming bodies
            return
        has_body = serialized.data != b""
        has_content_type = self._has_header("Content-Type", serialized.headers)
        if has_body and not has_content_type:
            serialized.headers["Content-Type"] = mime_type
class S3ResponseSerializer(RestXMLResponseSerializer):
    """
    The ``S3ResponseSerializer`` adds some minor logic to handle S3 specific peculiarities with the error response
    serialization.
    """
    SUPPORTED_MIME_TYPES = [APPLICATION_XML, TEXT_XML]
    def _serialize_r
        self,
        parameters: dict,
        response: HttpResponse,
        shape: Optional[Shape],
        shape_members: dict,
        operation_model: OperationModel,
        mime_type: str,
    ) -> None:
        header_params, payload_params = self._partition_members(parameters, shape)
        self._process_header_members(header_params, response, shape)
        ponses without the actual body.
        # Do not process the body payload in this case (setting a body could also manipulate the headers)
        # If the response is a redirection, the body should be empty as well
        if operation_model.http.get("method") != "HEAD" and not 300 <= response.status_code < 400:
            self._serialize_payload(
                payload_params, response, shape, shape_members, operation_model, mime_type
            )
        self._serialize_content_type(response, shape, shape_members, mime_type)
        self._prepare_additional_traits_in_rresponse, operation_model)
    def _serialize_error(
        self,
        error: ServiceException,
        response: HttpResp