[#451] Don't skip [Next]PartNumberMarker in response

In ListParts response we should always keep PartNumberMarker (even if it's zero)
and NextPartNumberMarker (even if response isn't truncated)

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/layer/multipart_upload.go

@@ -603,10 +603,10 @@ func (n *Layer) ListParts(ctx context.Context, p *ListPartsParams) (*ListPartsIn
 
 	if len(parts) > p.MaxParts {
 		res.IsTruncated = true
-		res.NextPartNumberMarker = parts[p.MaxParts-1].PartNumber
 		parts = parts[:p.MaxParts]
 	}
 
+	res.NextPartNumberMarker = parts[len(parts)-1].PartNumber
 	res.Parts = parts
 
 	return &res, nil