toJSON() serializes a pos object for a complex custom property

[andrewtelnov]

The following code will generate "pos" objects in question serialization:

  Serializer.addClass("optionstype",
    [
      { name: "itemValue" },
      { name: "colorId" },
      { name: "label:text", isLocalizable: true },
    ], undefined);
  Serializer.addProperty("question", { name: "options:optionstype" });
  Serializer.addProperty("question", { name: "optionsarray:optionstype[]" });