enable IE8 to set html into style element

fixes #2479 

Based on code ([and spec](http://jsfiddle.broadleafsystems.com/sLtph/7/)) suggested by @swhiteman in https://github.com/mootools/mootools-core/issues/2479, this PR fixes IE8 when setting html into style elements.

Specs added and [tested](https://travis-ci.org/SergioCrisostomo/mootools-core/builds/27441233).

I would just use `this.set('html', '');` it's BC with other versions anyway.