Make gallery columns fallback a string, as the accepted values are strings. see #21390.

git-svn-id: http://core.svn.wordpress.org/trunk@22341 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/media-views.js

@@ -1574,7 +1574,7 @@
 		settings: {
 			columns: {
 				accepts:  _.invoke( _.range( 1, 10 ), 'toString' ),
-				fallback: 3,
+				fallback: '3',
 				type:     'select'
 			},
 			link: {