Upgrade presentation 2 language properties

[stephenwf]

Source: http://iiif.io/api/presentation/2.1/example/fixtures/6/manifest.json

"description": [
  "This is one description", 
  {
    "@value": "This is another", 
    "@language": "en"
  }
], 

The "This is one description" is dropped during the upgrade process.