code: public static void serializeHeadersForMultipartRequests(
        HttpHeaders headers, StringBuilder logbuf, Logger logger, Writer writer) throws IOException {
      serializeHeaders(headers, logbuf, null, logger, null, writer);
    }
docs: Serializes headers to an {@link Writer} for Multi-part requests.
  
  @param headers HTTP headers
  @param logbuf log buffer or {@code null} for none
  @param logger logger or {@code null} for none. Logger must be specified if log buffer is
  specified
  @param writer Writer where HTTP headers will be serialized to or {@code null} for none
  @since 1.9