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

after creation: Uninitialized
  Status: 0
  StatusText: ""
  ResponseText: ""
  ResponseXML: null
  ResponseXML serialized: n/a
after setting onreadystatechange: Uninitialized
onreadystatechange: Open
  Status: 0
  StatusText: ""
  ResponseText: ""
  ResponseXML: null
  ResponseXML serialized: n/a
after open(): Open
  Status: 0
  StatusText: ""
  ResponseText: ""
  ResponseXML: null
  ResponseXML serialized: n/a
after overrideMimeType(): Open
after send(): Open
onreadystatechange: Sent
  Status: 200
  StatusText: "OK"
  ResponseText: ""
  ResponseXML: null
  ResponseXML serialized: n/a
onreadystatechange: Loaded
  Status: 200
  StatusText: "OK"
  ResponseText: ""
  ResponseXML: null
  ResponseXML serialized: n/a