use Object.prototype to avoid old IE's

Related info
[here](https://github.com/mootools/mootools-more/commit/0450a3d04bf5b769d38cb976ac7c3f9bf78cf4a8#commitcomment-5971036)
Merge pull request #1239 from SergioCrisostomo/ownProperty-fix

use Object.prototype to avoid old IE's