refer to http://stackoverflow.com/questions/896974/apache-is-not-sending-304-response-if-mod-deflate-and-addoutputfilterbytype-is

http://scarff.id.au/blog/2009/apache-304-and-mod_deflate-revisited/

Fix solution:

#add following to .htaccess or httpd.conf

<IfModule mod_headers.c>
RequestHeader  edit "If-None-Match" "^\"(.*)-gzip\"$" "\"$1\""
</IfModule>

Apache 2.4 issue "Apache is not sending 304 response (if mod_deflate and AddOutputFilterByType is enabled)"