[admin] Fixed VPN context in preview #57

Fortunately it was just a frontend JS issue.
The preview instance was getting the UUID of the Device
object instead of the Config object, and that prevented
the system from finding the associated VPN and fill the
context VPN keys correctly.

Fixes #57

django_netjsonconfig/static/django-netjsonconfig/js/preview.js

@@ -8,6 +8,9 @@ django.jQuery(function($) {
             fields = $(selectors, '#content-main form').not('#id_config_jsoneditor *'),
             $id = $('#id_id'),
             data = {};
+        // add id to POST data
+        // note: may be overridden by fields of OneToOne relation
+        if ($id.length) { data['id'] = $id.val() }
         // gather data to send in POST
         fields.each(function(i, field){
             var $field = $(field),
@@ -23,8 +26,6 @@ django.jQuery(function($) {
             }
             data[name] = $field.val();
         });
-        // add id to POST data
-        if ($id.length) { data['id'] = $id.val() }
         // show preview
         $.post(preview_url, data, function(html){
             inner.html($('#content-main div', html).html());