Use the native isArray whenever possible. See perf test by jdalton he…

…re: http://jsperf.com/isarray-vs-other Fixes #6825.

src/core.js

@@ -441,7 +441,7 @@ jQuery.extend({
 		return jQuery.type(obj) === "function";
 	},
 
-	isArray: function( obj ) {
+	isArray: Array.isArray || function( obj ) {
 		return jQuery.type(obj) === "array";
 	},