title: Laravel Excel is converting dates from heading into some numbers
body: <p>I have a problem which is connected with Laravel Excel. I have heading filled with dates for example: 2018-05-23. But when I read it with excel extension by using:</p>
  
  <pre><code> $data = Excel::load($request['file'], function($reader) {})-&gt;get();
  </code></pre>
  
  <p>The extension is not treating the heading dates as dates. Instead of that it converts the dates into some numbers - for example: <code>43243</code>.</p>
  
  <p>Here is <code>dd</code> of the sheet:</p>
  
  <p><a href="https://i.stack.imgur.com/4ikgE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4ikgE.png" alt="enter image description here"></a></p>
  
  <p>Important note: When the date is in a normal row (not in heading) then the date is converted well.</p>
  
  <hr>
  
  <p>Version of Laravel Excel (maatwebsite/excel) - <strong>2.1.0</strong><br>
  Version of Laravel - <strong>5.5</strong></p>