adding 304 support for if-Modified-Since request header, if a fresh cached...

adding 304 support for if-Modified-Since request header, if a fresh cached entity is present. backport from 2.1


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@104472 13f79535-47bb-0310-9956-ffa450edef68