Apply toLowerCase when reading featured articles

Consistently with getInterlanguageList(). Without this 'nds-NL'
will be handled incorrectly when the article in it is featured.

Bug: T143527
Change-Id: Ie7fe5a270cb3030899b6034512351d5d66d7be06

resources/js/ext.uls.compactlinks.js

@@ -378,7 +378,7 @@
 	 */
 	CompactInterlanguageList.prototype.filterByBadges = function () {
 		return $( '#p-lang' ).find( '[class*="badge"]' ).map( function ( i, el ) {
-			return $( el ).find( 'a' ).attr( 'lang' ); }
+			return $( el ).find( 'a' ).attr( 'lang' ).toLowerCase(); }
 		).toArray();
 	};