Fixes #1758

This is a fix for issue 1758: https://github.com/ivaynberg/select2/issues/1758

We use select2 at www.ecscapethecity.org and after some extensive user testing we figured it makes for a better user experience when the text a user entered is removed when closeOnSelect is false.

We want our users to select multiple tags easily - hence the closeOnSelect false - after a select has triggered the remaining text limits their view on other remaining tags. I have watched our testers hitting the delete key extensively because of this.