Fix Measure change serialization

As in commodities#show request Measure should expose the following
structure:

{
  measure_type: {
    id: '',
    description: ''
  }
}

Instead of having measure_type_description key.

Fixed/enhanced a few specs as well.
Merge pull request #123 from alphagov/60242196-fix-measure-change-serialization

Fix Measure change serialization