This adds/fixes the following things:
- Add missing KeyCount in result (fixes#734).
- Do not hard code MaxKeys to 1000.
- Truncate result if it has more than MaxKeys items. Set IsTruncated
and NextContinuationToken accordingly.
- Support the StartAfter parameter.
- Return Owner information only when FetchOwner=True is given.
- "Prefix" in response is now "" instead of None when omitted in request.
- "Delimiter" is now omitted from response when not given in request.