Ability to change the "no existing match" label, and selecting it does nothing

The input's value shouldn't be changed to "no existing match" if clicked, since it's not a valid item. I've fixed this.

Also, you can provide a "data-no-matches-label" attribute on the autocomplete input which specifies the text that will be shown if there are no matches.

<%= autocomplete_field_tag :user_name, '', autocomplete_user_name_users_path, :id_element => '#user_id', :data => { :no_matches_label => 'No matches found' } %>

For the future, we could have the label passed as a special hash value parameter.