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.
props adamsilverstein.

see #22919.


git-svn-id: https://develop.svn.wordpress.org/trunk@23247 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/js/media-editor.js

@@ -167,7 +167,7 @@
 				itemtag:    'dl',
 				icontag:    'dt',
 				captiontag: 'dd',
-				columns:    3,
+				columns:    '3',
 				size:       'thumbnail',
 				orderby:    'menu_order ID'
 			},