code: public TDHttpClient withHeaders(Multimap<String, String> headers)
      {
          Multimap<String, String> mergedHeaders = ImmutableMultimap.<String, String>builder()
                  .putAll(this.headers)
                  .putAll(headers)
                  .build();
          return new TDHttpClient(config, httpClient, objectMapper, mergedHeaders);
      }
docs: Get a {@link TDHttpClient} that uses the specified headers for each request. Reuses the same
  underlying http client so closing the returned instance will return this instance as well.
  
  @param headers
  @return