The form POST operation to S3 accepts a `success_action_status` field which can be set to 200/201/204, see [here](https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPOST.html). Localstack currently handles the `success_action_redirect` field [here](https://github.com/localstack/localstack/blob/master/localstack/services/s3/s3_listener.py#L518) and so we should look into handling the `success_action_status` field too.

The 201 from AWS responds with an XML document with some fields about the object that has just been created in AWS, see below.

```
<PostResponse>
    <Location>https://s3.amazonaws.com/link-to-the-file</Location>
    <Bucket>s3-bucket</Bucket>
    <Key>development/8614bd40-691b-4668-9241-3b342c6cf429/image.jpg</Key>
    <ETag>"32-bit-tag"</ETag>
</PostResponse>
```
return filters.get('S3Key', filters.get('Key', {}))
def prefix_with_slash(s):
    return s if s[0] == '/' else '/%s' % s
def get_event_message(event_name, bucket_name, file_name='testfile.txt', version_id=None, file_size=1024):
    
    bucket_name = normalize_bucket_name(bucket_name)
    return {
        'Records': [{
            'eventVersion': '2.0',
            'eventSource': 'aws:s3',
            'awsRegion': aws_stack.get_region(),
            'eventTime': timestamp(format=TIMESTAMP_FORMAT_MILLIS),
            'eventName': event_name,
            'userIdentity': {
                'principalId': 'AIDAJDPLRKLG7UEXAMPLE'
            },
            'requestParameters': {
                'sourceIPAddress': '127.0.0.1'  
            },
            'responseElements': {
                'x-amz-request-id': short_uid(),