void storeRequestIfModifiedSinceFor304Response(final HttpRequest request, final HttpResponse backendResponse) {
        if (backendResponse.getCode() == HttpStatus.SC_NOT_MODIFIED) {
            final Header h = request.getFirstHeader(HttpHeaders.IF_MODIFIED_SINCE);
            if (h != null) {
                backendResponse.addHeader(HttpHeaders.LAST_MODIFIED, h.getValue());
            }
        }
    }
For 304 Not modified responses, adds a "Last-Modified" header with the
value of the "If-Modified-Since" header passed in the request. This
header is required to be able to reuse match the cache entry for
subsequent requests but as defined in http specifications it is not
included in 304 responses by backend servers. This header will not be
included in the resulting response.
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CachingExecBase.java