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
fixes #23021 for 3.5


git-svn-id: http://core.svn.wordpress.org/branches/3.5@23281 1a063a9b-81f0-0310-95a4-ce76da25c4cd