When inserting a Gallery to be ordered by Date/Time use the post_date field...

When inserting a Gallery to be ordered by Date/Time use the post_date field for ordering rather than ID so that if the user/plugin has updated this to reflect the date/time when an image was captured we order correctly. Fixes #17485 props SergeyBiryukov.

git-svn-id: https://core.svn.wordpress.org/trunk@18596 1a063a9b-81f0-0310-95a4-ce76da25c4cd