Fix error with 'Add existing record' using record_select

In Active Scaffold 3.2.12 (possibly in d7d2a14c53df06d617439b89c9a83abc85b7f853) was introduced a bug, that fires javascript error on selecting a record from record_select in 'Add existing record'.

The reason is an URL, that in that version and above become an javascript Object (Ruby hash), not a String.

Affected versions: **rails-3.2** branch (`/frontends/default/views/_form_association_footer.html.erb:37`) version 3.2.12 and above and **master** branch (`app/views/active_scaffold_overrides/_form_association_footer.html.erb:37`).

There is a quick fix for master branch.