[SeaMonkey] Can't detect already opened built-in private windows

[Infocatcher]

See 2a33253 commit.
The logic was changed in release version. :(
chrome://navigator/content/navigator.js, SeaMonkey 2.19:

  var lc = window.QueryInterface(Components.interfaces.nsIInterfaceRequestor)
                 .getInterface(Components.interfaces.nsIWebNavigation)
                 .QueryInterface(Components.interfaces.nsILoadContext);
  if (lc.usePrivateBrowsing) {
    gPrivate = window;
    document.documentElement.removeAttribute("windowtype");

So we should check for window.location.href instead of window.document.documentElement.getAttribute("windowtype").
Also we probably can check only location and remove fix for #61.