title: Is it possible to restrict fields on included associations with ActiveModelSerializers?
body: <p>The guide for the latest version (0.10.7) documents a <code>include</code> option, that restricts what associated serializers' data will be included in the json output, and a <code>fields</code> option, that restricts what attributes of the top level serializer will be included, but is there a way to restrict what fields on the associated serializer are included? This seems to be all or nothing.</p>
  
  <p>This <a href="https://github.com/rails-api/active_model_serializers/issues/1849" rel="nofollow noreferrer">GitHub issue</a> includes the following example syntax that seemingly does exactly what I want, but it doesn't seem to work with the latest version:</p>
  
  <p><code>render json: @blog, include: 'posts.category, posts.author.address', fields: { posts: { category: [:name], author: [:id, :name] } }</code></p>