title: Removing relations is not being audited by audited gem
body: <p>I'm using <a href="https://github.com/collectiveidea/audited/tree/4.2-stable#associated-audits" rel="nofollow">Associated Audits</a> on a <code>has_many through</code> relation with Collective Idea's <a href="https://github.com/collectiveidea/audited" rel="nofollow">audited</a> gem. I see <code>create</code> audits for the <code>through</code> model being added, but I do not see any audits when that relation is removed.</p>
  
  <p>Here are my 3 models. A <code>Post</code> can be in multiple <code>Categories</code>.</p>
  
  <p><strong>app/models/post.rb</strong></p>
  
  <pre><code>class Post &lt; ActiveRecord::Base
    audited
    has_associated_audits
  
    has_many :categorizations, dependent: :destroy
    has_many :categories, through: :categorizations
  end
  </code></pre>
  
  <p><strong>app/models/category.rb</strong></p>
  
  <pre><code>class Category &lt; ActiveRecord::Base
    audited
    has_associated_audits
  
    has_many :categorizations, dependent: :destroy
    has_many :posts, through: :categorizations
  end
  </code></pre>
  
  <p><strong>app/models/categorization.rb</strong></p>
  
  <pre><code>class Categorization &lt; ActiveRecord::Base
    audited
    audited associated_with: :post
    audited associated_with: :category
  
    belongs_to :category
    belongs_to :post
  end
  </code></pre>
  
  <p>My <code>Post</code> form has a pile of checkboxes for categorization:</p>
  
  <pre><code>&lt;%= f.association :categories, as: :check_boxes, collection: Category.order(:name), label_method: :name, value_method: :id, label: false %&gt;
  </code></pre>
  
  <ul>
  <li>When I edit an existing <code>Post</code> and <strong>check</strong> a box for a <code>Category</code>, I <strong>do</strong> see a new audit entry with a <code>create</code> value in the audit's action field.</li>
  <li>When I edit an existing <code>Post</code> and <strong>uncheck</strong> a box for a <code>Category</code>, I <strong>do not</strong> see a new audit entry.</li>
  <li><p>I do see <code>destroy</code> audits for both the <code>Post</code> and the <code>Categorization</code> auditable_type fields when I delete a <code>Post</code>, so that aspect works nicely.</p>
  
  <ol>
  <li>Can audited track those deselections? If so, how?</li>
  <li>Is there anything obviously bad/wrong about my audited setup in the models above? There's no <code>has_many through</code> documentation to follow, so I guessed a bit.</li>
  </ol></li>
  </ul>