### 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
d(
                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_payload(
        self,
        parameters: dict,
        response: HttpResponse,
        shape: Optional[Shape],
        shape_members: dict,
        operation_model: OperationModel,
        mime_type: str,
    ) -> None:
        """
        Serializes the given payload.
        :param parameters: The user input params
        :param response: The final serialized HttpResponse
        :param shy" response)
        :param shape_members: The members of the output struct shape
        :param operation_model: The specification of the operation of which the response is serialized here
        :param mime_type: Mime type which should be used to encode the payload
        :return: None - the given `serialized` dict is modified
        """
        if shape is None:
            return
        payload_member = shape.serialization.get("payload")
        # If this shape is defined as being an event, we need to search for the payload member
        if not payload_member and shape.serialization.get("event"):
            for member_name, member_shape in shape_members.items():
                # Try to find the first shape which is marked as "eventpayload" and is given in the params dict
                if member_shape.serialization.get("eventpayload") and parameters.get(member_name):
                    payload_member = member_name
                    break
        if payload_member is not None and shape_members[payload_member].type_name in [
            "blob",
            "string",
        ]:
            # If it's streaming, then the body is just the value of the payload.
            body_payload = parameters.get(payload_member, b"")
            body_payload = _payload(body_payload)
            response.set