title: Django Mongoengine leading whitespace being stripped
body: <p>I have a small issue that I'm struggling to find a solution on.</p>
  
  <p>Currently when I receive the result from a POST with JSON data back from Django(1.11.5), all leading whitespaces in some of the values are completely gone but all in between the words saved.</p>
  
  <p>Is there a setting for MongoEngine that stops this on StringFields?</p>
  
  <p>For example:
  I send with requests in Python(3.6.2):</p>
  
  <pre><code>{'foo': '  bar'}
  </code></pre>
  
  <p>I get back from the request content:</p>
  
  <pre><code>{'foo': 'bar'}
  </code></pre>
  
  <p>I'm using MongoEngine (0.14.3), django-restframwork-mongoengine (3.3.1) and viewsets.ModelViewSet btw :)</p>