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.


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@27977 602fd350-edb4-49c9-b593-d223f7449a82