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>
```
xml_with_last_modified = ('<?xml version="1.0" encoding="UTF-8"?>'
                                  '<ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">'
                                  '  <Name>thanos/Name>'
                                  '  <Contents>'
                                  '    <LastModified>2019-05-27T19:00:16.663Z</LastModified>'
                                  '  </Contents>'
                                  '</ListBucketResult>'
                                  )
        xml_without_last_modified = ('<?xml version="1.0" encoding="UTF-8"?>'
                                     '<ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">'
                                     '  <Name>thanos/Name>'
                                     '  <Contents>'
                                     '    <NotLastModified>2019-05-27T19:00:16.663Z</NotLastModified>'
                                     '  </Contents>'
                                     '</ListBucketResult>'
                                     )
        
        response = Response()
        s3_listener.append_last_modified_headers(response, content=xml_with_last_modified)
        self.assertEqual('Mon, 27 May 2019 19:00:16 GMT', response.headers.get('Last-Modified', ''))
        
        
        
        response = Response()
        s3_listener.append_last_modified_headers(response, content=xml_without_last_modified)
        self.assertNotEqual('No header', response.headers.get('Last-Modified', 'No header'))
        response = Response()
        s3_listener.append_last_modified_headers(response)
        self.assertNotEqual('No header', response.headers.get('Last-Modified', 'No header'))