code: protected boolean isNotModified(Resource resource, SlingHttpServletRequest request,
        SlingHttpServletResponse response) throws IOException {
      // check resource's modification date against the If-Modified-Since header and send 304 if resource wasn't modified
      // never send expires header on author or publish instance (performance optimization - if medialib items changes
      // users have to refresh browsers cache)
      return CacheHeader.isNotModified(resource, request, response, false);
    }
docs: Checks if the resource was modified since last request
  @param resource Resource pointing to nt:file or nt:resource node
  @param request Request
  @param response Response
  @return true if the resource is not modified and should not be delivered anew
  @throws IOException