`ValuesField` fields don't render properly on admin pages

## Bug report

### What is the current behavior?
`ValuesField` fields of models don't render properly on admin pages (e.g. `input_values` and `output_values`). They just show the class name.

### What is the expected behavior?
The actual values stored in the `ValuesField` should show. 

### Proposed Solution
The problem might be to do with the widget or the field class itself.
