Last-modified header set to today when cache is disabled

wp-optimize sends a Last-Modified header set to the current datetime for all pages and posts. This looks to be happining when minify is enabled (regardless of minify HTML setting), but caching is not. The header is sent in WP_Optimize_Minify_Front_End::add_caching_headers

Last-modified should reflect the content’s modified date, but if that’s out of scope for wp-optimize and you must send a last-modified header, it should at least check to see if the last-modified header has already been set so it doesn’t interfere with other attempts to send a proper header.