### 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
ef _serialize_type_structure(
        self, body: dict, value: dict, shape: StructureShape, key: Optional[str], mime_type: str
    ):
        if value is None:
            return
        if shape.is_document_type:
            body[key] = value
        else:
            if key is not None:
                # If a key is provided, this is a result of a recursive
                # call, so we need to add a new child dict as the value
                # of the passed in serialized dict.  We'll then add
                # all the structure members as key/vals in the new serialized
                # dictionary we just created.
                new_serialized = {}
                body[key] = new_serialized
                body = new_serialized
            members = shape.members
            for member_key, member_value in value.items():
                if member_value is None:
                    continue
                try:
                    member_shape = members[member_key]
                except KeyError:
                    LOG.warning(
                        "Response object %s contains a member which is not specified: %s",
                        shape.name,
                        member_key,
                    )
                    continue
                if "name" in member_shape.serialization:
                    
                smber_key, mime_type)
    def _serialize_type_map(
        self, body: dict, value: dict, shapepe: str
    ):
        if value is None:
            return
        map_obj = {}
        body[key] = map_obj
        for sub_key, sub_value in value.items():
            if sub_value is not None:
                self._serialize(map_obj, sub_value, shape.value, sub_key, mime_type)
    def _serialize_type_list(
        self, body: dict, value: list, shape: ListShape, key: str, mime_type: str
    ):
        if value is None:
            return
        list_obj = []
        body[key] = list_obj
        for list_item in value:
            if list