docs: private RequestTemplate appendHeader(String name, Iterable<String> values) {
      if (!values.iterator().hasNext()) {
        /* empty value, clear the existing values */
        this.headers.remove(name);
        return this;
      }
      this.headers.compute(name, (headerName, headerTemplate) -> {
        if (headerTemplate == null) {
          return HeaderTemplate.create(headerName, values);
        } else {
          return HeaderTemplate.append(headerTemplate, values);
        }
      });
      return this;
    }