Fix of recognition of ArrayBuffer parameters

If js-sha3 is loaded using 'webpack' loader then the top-most
fuction is not called with global object (`global` in node,
`window` in browsers) and thus `root.ArrayBuffer` is undefined.

This patch introduces usage of `ArrayBuffer` without `root`
prefix similary to how e.g. `Uint8Array` is already used.

src/sha3.js

@@ -106,7 +106,7 @@
 
   Keccak.prototype.update = function (message) {
     var notString = typeof message != 'string';
-    if (notString && message.constructor == root.ArrayBuffer) {
+    if (notString && message instanceof ArrayBuffer) {
       message = new Uint8Array(message);
     }
     var length = message.length, blocks = this.blocks, byteCount = this.byteCount,