XmlHttpRequest对象封装的函数，支持ie和firefox

<script>function createXMLHttp(){    var ret = null;    try {        ret = new ActiveXObject('Msxml2.XMLHTTP');    }    catch (e) {        try {            ret = new ActiveXObject('Microsoft.XMLHTTP');        }        catch (ee) {            ret = null;        }    }    if (!ret && typeof XMLHttpRequest != 'undefined')        ret = new XMLHttpRequest();    return ret;}</script>

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