In multi, autoselect the next option on select.

When you hit enter twice within a multiselect without closeOnSelect, you end up selecting the option twice. This is because the option, after being added the first time, is still considered "selected."

https://github.com/ivaynberg/select2/issues/243

Our solution is to select the next option automatically. This is a behavioral change, but we do believe it to be the right one; if a user hits enter multiple times, it is most likely because the user wants to select multiple items.