title: Laravel 5.2+ date before and after date validation not working
body: <p>This works</p>
  
  <pre><code>'expected_at' =&gt; 'date|after:"2016-04-09 10:48:11"',
  </code></pre>
  
  <p>And this works</p>
  
  <pre><code>$rules['expected_at'] = 'date|after:'.$opportunity-&gt;created_at;
  </code></pre>
  
  <p>This does not work</p>
  
  <pre><code>'expected_at' =&gt; 'date|after:created_at',
  </code></pre>
  
  <p>The "created_at" value in the database is exactly the following</p>
  
  <pre><code>2016-04-09 10:48:11
  </code></pre>
  
  <p>Note the form is passing the expected_at date to the validator in the following format</p>
  
  <pre><code>2016-04-09
  </code></pre>
  
  <p>I assume that this means you cannot directly reference a model field in a validator?</p>