解决了IE8不支持数组的indexOf方法

ie在过去给我们添了很多坑。

if (!Array.prototype.indexOf) {
    Array.prototype.indexOf = function(elt /*, from*/ ) {
        var len = this.length >>> 0;
        var from = Number(arguments[1]) || 0;
        from = (from < 0) ? Math.ceil(from) : Math.floor(from);
        if (from < 0)
            from += len;
        for (; from < len; from++) {
            if (from in this &&
                this[from] === elt)
                return from;
        }
        return -1;
    };
}