# Type of request: This is a bug report

# Detailed description
S3 list buckets returns a different format for CreationDate than the one expected by the Java SDK and parsing fails. Using Java SDK 2.13.39. Also verified w/ .40, .38, .30 and .20. Will check previous Docker releases for localstack.

``Unable to unmarshall response (Text '2020-06-17T15:26:46.032838+00:00' could not be parsed at index 19). Response Code: 200, Response Text: 
software.amazon.awssdk.core.exception.SdkClientException: Unable to unmarshall response (Text '2020-06-17T15:26:46.032838+00:00' could not be parsed at index 19). Response Code: 200, Response Text: <truncated...>
``

## Expected behavior
Return CreationDate in same format as the actual S3 API call.

## Actual behavior
Returns CreationDate in slightly different format from the actual S3 API call. Fractional seconds are included.

# Steps to reproduce
Run "aws s3api list-buckets" vs "aws s3api list-buckets --endpoint-url http://localstack:4566" and note different date format for CreationDate.

## Command used to start LocalStack
docker-compose:
```
  localstack:
    image: localstack/localstack
    ports:
      - '4566:4566'
    environment:
      SERVICES: s3
      DEBUG: 1
```
## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)
See steps to reproduce.
"""Add Last-Modified header with current time
    (if the response content is an XML containing <LastModified>, add that instead)"""
    time_format = '%a, %d %b %Y %H:%M:%S GMT'  
    try:
        if content:
            last_modified_str = re.findall(r'<LastModified>([^<]*)</LastModified>', content)
            if last_modified_str:
                last_modified_str = last_modified_str[0]
                last_modified_time_format = dateutil.parser.parse(last_modified_str).strftime(time_format)
                response.headers['Last-Modified'] = last_modified_time_format
    except TypeError as err:
        LOGGER.debug('No parsable content: %s' % err)
    except ValueError as err:
        LOGGER.error('Failed to parse LastModified: %s' % err)
    except Exception as err:
        LOGGER.error('Caught generic exception (parsing LastModified): %s' % err)
    
    try:
        if response.headers.get('Last-Modified', '') == '':
            response.headers['Last-Modified'] = datetime.datetime.now().strftime(time_format)
    except Exception as err:
        LOGGER.error('Caught generic exception (setting LastModified header): %s' % err)
def append_list_objects_marker(method, path, data, response):