Don't include the default columns="3" in a gallery shortcode.

Shortcode attributes are strings; fix a variable type error in wp.media.gallery defaults.

Merges [23247] to the 3.5 branch.

props adamsilverstein.
fixes #22919.



git-svn-id: https://core.svn.wordpress.org/branches/3.5@23248 1a063a9b-81f0-0310-95a4-ce76da25c4cd