title: DjangoRestFramework ModelSerializer DateTimeField only converting to current timezone at object creation
body: <p>Update: This behaviour was recognized as a bug and resolved in django-rest-framework 3.7. Serializers will now consistently display times according to the server's timezone: <a href="https://github.com/tomchristie/django-rest-framework/issues/3732#issuecomment-267635612" rel="nofollow noreferrer">https://github.com/tomchristie/django-rest-framework/issues/3732#issuecomment-267635612</a></p>
  
  <p>I have a Django project where I expect the user to be in a certain timezone. I have <code>TIME_ZONE = 'Asia/Kolkata'</code> and <code>USE_TZ = True</code> in my settings.</p>
  
  <p>I have a model that includes a datetimefield. When I first create the object, the modelserializer gives datetimes with a trailing <code>+5:30</code>. Annoyingly, datetimes with <code>auto_now_add=True</code> give UTC datetimes with a trailing <code>Z</code>. I fixed this by making the field's default a callable for the current time.</p>
  
  <p>If I serialize the object again at any point, all datetimes are in UTC with a trailing <code>Z</code>. From the Django documentation, I would expect the serializer to use the current timezone, which defaults to the default timezone set by <code>TIME_ZONE = 'Asia/Kolkata'</code>. I have checked the current timezone in my view with <code>get_current_timezone_name()</code> and it is <code>'Asia/Kolkata'</code>. I have even tried using <code>activate('Asia/Kolkata')</code> in my view, but times are still being returned in UTC.</p>
  
  <p>Note that all the times are correct (the UTC times are 5:30 hours earlier), its just that I would expect for the times to be converted. All datetimes are stored in the DB as UTC times as expected.</p>