Pass only strings to setProperty as the value argument on IE9 requires D...

An error occured on IE9 when trying to execute:

    move('.login-container .header')
        .duration(400)
        .ease('in')
        .set('opacity', 0)
        .x(-400)
        .end();

So I searched for the problem and found out that IE9 does not like it when passing a DOMNumber (opacity 0) into the style.setProperty function.
http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleDeclaration-setProperty