Fix #2428 IE8: Fx.Morph percentage unit issue

This pull request fixes the unit issue in the Fx.CSS class.
Indeed, we can not morph a property using a specific unit (let say %) and a color property at the same time.
This example fails:

el.set("morph", {
    "duration": 1000,
    "unit": "%"
});
el.get("morph").start({
    "height": "50",
    "background-color": "#00FF00"
});

The animation on the background-color does not work.