Use type as key for linked resources

If type is `author` but the association is called `writer`, the linked
resource key should be called `authors`, e.g

    {
      ...
      linked: {
        authors: [{
          ...
        }]
      }
      ...
    }
Merge pull request #728 from rails-api/linked-resources-use-type-as-key

Use type as key for linked resources