title: Filling public_activity parameters with model instance attributes
body: <p>I am trying to fill the <code>parameters</code> attribute of my activity model (from the public_activity gem), but without success.</p>
  
  <pre><code>class Project &lt; ActiveRecord::Base
    include PublicActivity::Model
  
    tracked owner: -&gt;(controller, model) { controller &amp;&amp; controller.current_user },
            parameters: -&gt;(controller, model){
              {foo: 'bar'}
            }
  end
  </code></pre>
  
  <p>The first problem is that the parameters hash is always empty when I update a project. The second problem is, that <code>-&gt;(controller, model)</code> receives the controller and the model class, but not the instance. My aim is to pu two fields from the changeset on update into the parameters (an id and a string).</p>
  
  <p>How can I achieve this?</p>