code: private static Property[] getDatastreamHeaders(String pid, Datastream ds) {
          Property[] result = new Property[3];
          result[0] = new Property(HttpHeaders.ACCEPT_RANGES,"bytes");
          result[1] = new Property(HttpHeaders.ETAG, Datastream.defaultETag(pid, ds));
          result[2] = new Property(HttpHeaders.LAST_MODIFIED, DateUtil.formatDate(ds.DSCreateDT));
          return result;
      }
docs: Content-Length is determined elsewhere
  Content-Type is determined elsewhere
  Last-Modified
  ETag
  @param ds
  @return