Fix issue when using modified Array.prototype for IE8 backports

When backporting Array compatibility to IE, functions my be added such as:

if (!Array.prototype.diff) {
    Array.prototype.diff = function(a) {
        return this.filter(function(i) {return !(a.indexOf(i) > -1);});
    };
}

(see Compatibility in MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf?redirectlocale=en-US&redirectslug=JavaScript%2FReference%2FGlobal_Objects%2FArray%2FindexOf)

The for (index in some_array) will include non "hasOwnProperty" indices and thus break with this backport.

Using for (var i = 0; i < some_array.length; i++) syntax will fix this.