If-Modified-Since header does not works (source)

View source for If-Modified-Since header does not works

It does not reseive 304 code. I check it on https://last-modified.com/en/ and https://bertal.ru/index.php?a6666810/https://wackowiki.org/doc/HomePage#h .
In my istallation R5.5.10 I fix it by inserting php code provided on Last-Modified.com to first line of  \theme\_common\_header.php 
Probably there is more beautiful solution.