Resolves a bug where a post save will be reported as failed if the post includes any meta keys the current user does not have authorization to update, even when those meta values are unchanged.
Write authorization is now checked for a meta key only when the value of that key has changed, so that passing a REST response back unchanged will not cause failures.
Authorization is only needed when data will be updated.
Props ckoerner, TimothyBlynJacobs, spacedmonkey
Built from https://develop.svn.wordpress.org/trunk@56075
git-svn-id: http://core.svn.wordpress.org/trunk@55587 1a063a9b-81f0-0310-95a4-ce76da25c4cd
0d9723a44e