Use Array.isArray, when possible.

unpacked/MathJax.js

@@ -155,7 +155,7 @@ MathJax.cdnFileVersions = {};  // can be used to specify revisions for individua
     })
   });
   
-  BASE.Object.isArray = function (obj) {
+  BASE.Object.isArray = Array.isArray || function (obj) {
     return Object.prototype.toString.call(obj) === "[object Array]";
   };