Use native dataset if available

Fixes TypeError in strict mode (`dataset` is readonly)

Author: samme

src/loader/filetypes/HTML5AudioFile.js

@@ -148,7 +148,12 @@ var HTML5AudioFile = new Class({
         for (var i = 0; i < instances; i++)
         {
             var audio = new Audio();
-            audio.dataset = {};
+
+            if (!audio.dataset)
+            {
+                audio.dataset = {};
+            }
+
             audio.dataset.name = this.key + ('0' + i).slice(-2);
             audio.dataset.used = 'false';