Prevent extra request from being sent on selection

Fix for https://github.com/geoadmin/mf-geoadmin3/issues/1623

It seems that this is a known issue in typeahead and that it has been fixed in a later version.
https://github.com/twitter/typeahead.js/commit/95e74317fdcd4f1cacbc4bd2c15d06efcecbb24e

I used a workaround in `beforeSend` similar to what we have for coordinates and couldn't find any drawback so far.
Just curious: you need to use decodeURIComponent here because the selected text can contain html snippets? (<b> for example) Or is there another reason?