$.ajax({
    beforeSend: function(req) {
        req.setRequestHeader("Accept", "text/xml");
    },
    type: "GET",
    url: "[proper url]",
    contentType: "text/plain; charset=utf-8",
    dataType: ($.browser.msie) ? "text" : "xml",
    username: '---',
    password: '-------',
    success: function(data) {
        var xml;
        if (typeof data == "string") {
            alert("Data is string:" + data);
            xml = new ActiveXObject("Microsoft.XMLDOM");
            xml.async = false;
            xml.loadXML(data);
        } else {
            xml = data;
            alert("Data is not string:" + $(xml).text());
        }
        // Returned data available in object "xml"
        //alert("Status is: " + xml.statusText);
        $("#ingest_history").html($(xml).text());
    }
});

setRequestHeader('Accept', '')

setRequestHeader('Accept', null)

$.ajax({
    headers: {
        Accept : "text/plain; charset=utf-8",
        "Content-Type": "text/plain; charset=utf-8"
    }
    data: "data",
    success : function(response) {
        ...
    }
})

$.ajax({
    dataType: ($.browser.msie) ? "text" : "xml",
    accepts: {
        xml: "text/xml",
        text: "text/xml"
    }
});