Edit model fix:

Solves the issue with showing previous content for single edit.

Manually forced the response for single edit request to match action form response. Actually wraps model fields in a "values" keyed hash. E.g. compare : 

Pause action response:
```json
{
  title: null,
  prompt: null,
  values: {
    pause_reason: {
      value: null,
      choices: null
    },
    hide: {
      value: true,
      choices: null
    }
  }
}
```

Current edit response:
```json
{
  _model: "Order",
  _label: "Order f9cde",
  id: "52e194469bea15e113000004",
  values: {
    internal_note: {
      value: "prova1"
    }
  }
}
```

Previous edit response:
```json
{
  _model: "Order",
  _label: "Order f9cde",
  id: "52e194469bea15e113000004",
  internal_note: {
    value: "prova1"
  }
}
```

This solves the issue with single edits. I tried solving entire object edit too, and thought it had to do with the same representation. If I use ```model_config.flat_fields``` on line 36 I get the correct wrapping, but unfortunately again only the explicit edit field is shown in the form. Am I understanding this incorrectly, maybe only ```:edit``` fields are to be shown on the form?