model idAttribute does not work because "id" is hardcoded in DropdownListView.render

[DougLeary]

DropdownListView.render() fails if the model's id property is not called "id".
The following change fixes this by using the model's idAttribute instead of literal "id":

 render: function() {
     var html = this.collection.reduce(function(str, model) {

•        return str + "<li data-id=\""+ model.get("id") +"\">"+ model.get(this.title_field) +"</li>";
  

•       return str + "<li data-id=\""+ model.id +"\">"+ model.get(this.title_field) +"</li>";
   }, "", this);