title: Set DatetimeField format of django rest framework configuration
body: <p>I try to use DRF serializers to serialize a model object. I find the DatetimeField in object won't output <code>"2015-10-21T09:28:53.769000Z"</code> of the ISO-8601 format</p>
  
  <p>I lookup DRF document why I can't output ISO-8601 format. According to <a href="http://www.django-rest-framework.org/api-guide/fields/#datetimefield" rel="noreferrer">datetimefield</a> says:</p>
  
  <blockquote>
    <p><code>format</code> - A string representing the output format. If not specified, this defaults to the same value as the DATETIME_FORMAT settings key, which will be 'iso-8601' unless set. Setting to a format string indicates that to_representation return values should be coerced to string output. Format strings are described below. Setting this value to None indicates that Python</p>
  </blockquote>
  
  <p>It means It's default to output <code>iso-8601</code> format if I never set <code>DATETIME_FORMAT</code> argument? Not yet, it's still no change.</p>
  
  <p>When I try to write setting of django project as the following:</p>
  
  <pre><code>REST_FRAMEWORK = {
      'DATETIME_FORMAT': "iso-8601",
  }
  </code></pre>
  
  <p>or I write in DatetimeField argument as the following:</p>
  
  <pre><code>class UserSerializer(...):
      last_login = DatetimeField(format='iso-8601')
      class Meta:
          model = User
          fields = ('email', 'displayname', 'is_active',
                    'date_joined', 'last_login')
  </code></pre>
  
  <p>It's still no change again.</p>
  
  <p>Anyone know how to set it?</p>