docs: private Map<String, String> getCustomHeaders(HttpHeaders httpHeaders, Map<String, Object> mappedHeaders) {
          Map<String, String> customHeaders = new HashMap<>();
  
          for (Map.Entry<String, List<String>> header : httpHeaders.entrySet()) {
              if (!mappedHeaders.containsKey(header.getKey())) {
                  customHeaders.put(header.getKey(), StringUtils.collectionToCommaDelimitedString(header.getValue()));
              }
          }
  
          return customHeaders;
      }