Better checks for contributors when saving posts.

Merges [27976] from the 3.8 branch to the 3.7 branch.

props dd32, kovshenin, plocha.
fixes #27452.

Built from https://develop.svn.wordpress.org/branches/3.7@27977


git-svn-id: http://core.svn.wordpress.org/branches/3.7@27807 1a063a9b-81f0-0310-95a4-ce76da25c4cd