Make sure the $parent argument of wp_insert_attachment() still works as expected after [28579].

prop jesin, dikiy_forester.
fixes #29646 for trunk.
Built from https://develop.svn.wordpress.org/trunk@29745


git-svn-id: http://core.svn.wordpress.org/trunk@29519 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge pull request #17 from WordPress/master

Update