Fix access to inherited properties

When loading a JSON level melonJS crashes with the following error:

     Uncaught TypeError: Object function () {
       var newObj = (this instanceof Array) ? [] : {};
       for (i in this) {
         if (i == 'clone') continue;
         if (this[i] && typeof this[i] == "object") {
           newObj[i] = this[i].clone();
         } else newObj[i] = this[i]
       } return newObj;
     } has no method 'isBoolean' melonJS.js:9848
     me.TMXUtils.api.applyTMXPropertiesFromJSON melonJS.js:9848
     me.TMXMapReader.extend.readJSONMap melonJS.js:12230
     me.TMXMapReader.Object.extend.readMap melonJS.js:11819
     me.levelDirector.obj.loadLevel melonJS.js:12483
     me.ScreenObject.extend.onResetEvent playscreen.js:10
     me.ScreenObject.me.Renderable.extend.reset melonJS.js:5073
     _switchState melonJS.js:5344
     Function.defer melonJS.js:554 

Solved the bug by cheking if property is inherited.