title: touch: true not triggered when object is deleted
body: <p>I have two models - Post and Comment. I'm using fragment caching to cache each <code>post</code> like so:</p>
  
  <pre><code>- @posts.each do |post|
    - cache post do
      = render 'posts/post', post: post
  </code></pre>
  
  <p><strong>post.rb</strong></p>
  
  <pre><code>class Post &lt; ActiveRecord::Base
    belongs_to :user, touch: true
    has_many :comments, dependent: :destroy
  end
  </code></pre>
  
  <p>Now my <code>comments.rb</code> file is as follows:</p>
  
  <pre><code>class Comment &lt; ActiveRecord::Base
    belongs_to :user
    belongs_to :post, touch: true
  end
  </code></pre>
  
  <p>This works as expected. Whenever I add a new comment to a post, the cache key of that particular post expires. However, when I <strong><em>delete</em></strong> a comment from a post, the cache key of that post doesn't expire. What gives?</p>