fix resource registry

Before:
```
In [8]: deform.widget.default_resource_registry.registry

...

 'chosen': {None: {'css': (('jquery_chosen/chosen.css',
     'chosen_bootstrap.css'),),
   'js': (('jquery_chosen/chosen.jquery.js',),)}},
```

After:

```
 'chosen': {None: {'css': ('jquery_chosen/chosen.css', 'chosen_bootstrap.css'),
   'js': ('jquery_chosen/chosen.jquery.js',)}},
```