code: private void appendHeaders(HttpHeaders headers, StringBuilder builder) {
          for (Entry<String, List<String>> headerEntry : headers.entrySet()) {
              builder.append(headerEntry.getKey());
              builder.append(":");
              builder.append(StringUtils.arrayToCommaDelimitedString(headerEntry.getValue().toArray()));
              builder.append(NEWLINE);
          }
      }
docs: Append Http headers to string builder.
  @param headers
  @param builder