function getXHR() {
  if (root.XMLHttpRequest
    && ('file:' != root.location.protocol || !root.ActiveXObject)) {
    return new XMLHttpRequest;
  } else {
    try { return new ActiveXObject('Microsoft.XMLHTTP'); } catch(e) {}
    try { return new ActiveXObject('Msxml2.XMLHTTP.6.0'); } catch(e) {}
    try { return new ActiveXObject('Msxml2.XMLHTTP.3.0'); } catch(e) {}
    try { return new ActiveXObject('Msxml2.XMLHTTP'); } catch(e) {}
  }
  return false;
}
Default MIME type map.

    superagent.types.xml = 'application/xml';