Check caps for both old and new meta keys when changing the key for a mid. ...

Check caps for both old and new meta keys when changing the key for a mid.  Properly handle slashes when checking meta caps.  Props xknown. see #17850

git-svn-id: https://core.svn.wordpress.org/trunk@18449 1a063a9b-81f0-0310-95a4-ce76da25c4cd