Test for bug 5924 - zero-length responses to XMLHTTPRequest mishandled.

after creation: Uninitialized
ResponseText: ""
after setting onreadystatechange: Uninitialized
after open(): Open
after overrideMimeType(): Open
after send(): Loaded
  Status: 200
  StatusText: "OK"
  ResponseText: ""
  ResponseXML: null
  ResponseXML serialized: n/a