Fix Measure change serialization

This change is for https://www.pivotaltracker.com/story/show/60242196

As in commodities#show request Measure should expose the following structure:

```json
{
  measure_type: {
    id: '',
    description: ''
  }
}
```
Instead of having `measure_type_description key`.

This prevent certain goods code changes from working.

Fixed/enhanced a few specs as well.