Try not to send Last-Modified, even with an empty value. Some servers interpret an empty value as the epoch.

Props nacin, slene, SergeyBiryukov, andy
see #23021 for trunk


git-svn-id: https://core.svn.wordpress.org/trunk@23267 1a063a9b-81f0-0310-95a4-ce76da25c4cd