@Internal
    public static void normalizeElements(final MessageHeaders message, final String headerName, final Consumer<String> consumer) {
        // User-Agent as a special case due to its grammar
        if (headerName.equalsIgnoreCase(HttpHeaders.USER_AGENT)) {
            final Header header = message.getFirstHeader(headerName);
            if (header != null) {
                consumer.accept(header.getValue().toLowerCase(Locale.ROOT));
            }
        } else {
            normalizeElements(message.headerIterator(headerName), consumer);
        }
    }
Returns all variant names contained in {@literal VARY} headers of the given message.

@since 5.4
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CacheKeyGenerator.java