title: serializer.data is missing some of the data
body: <p>Context: I am having problem accessing fields which are validated by nested serializers.
  I have a very sample model as shown below. 
  For 2 of the fields I have their specific serializers. When I try to access the data it returns all the fields except the one validated by the specific serializers. </p>
  
  <p>Models looks like this</p>
  
  <pre><code>class Sampler(models.Model):
    sample_name = models.CharField(unique=True, max_length=100)
    sampling_by = JSONField(max_length=100)
    extractions = JSONField(max_length=100)
    max_samples = models.IntegerField(default=100)
  </code></pre>
  
  <p>Serializers</p>
  
  <pre><code>class ExtractionsSerializer(serializers.BaseSerializer):
     table_name = serializers.CharField()
     extraction_type = serializers.ChoiceField(["ABC"])
     dependencies = serializers.ListField(child=RecursiveField(), allow_empty=True, required=False)
  
  
  class SamplingBySerializer(serializers.BaseSerializer):
  """
      Validate sampling_by
  """
      def to_internal_value(self, samples):
        methods = ["ABC"]
        sampling_not_supported = [sample for sample in samples
                                if sample['by'] not in methods]
         if sampling_not_supported:
            raise ValidationError("{} not in {}".format(sampling_not_supported, methods))
  
  class SamplerSerializer(serializers.ModelSerializer):
      extractions = ExtractionsSerializer(read_only=True)
      sampling_by = SamplingBySerializer(read_only=True)
  
      class Meta:
        model = Sampler
        fields = ('sample_name', 'database', 'schema', 'sampling_by', 'extractions', 'max_samples')
  </code></pre>
  
  <p>Now I do</p>
  
  <pre><code>data = {
  "database": "postgresql://..",
  "sampling_by":[{
      "by":"ABC",
      "value": ["l32=turn_the"]
  }],
  "max_samples":3,
  "extractions" : [{
      "table_name": "person", 
      "extraction_type": "ABC"
  }]
  }
  sampler = SamplerSerializer(data=data)
  sampler.is_valid() #returns True
  sampler.data =&gt; does not contain data of the nested fields. Like the `sampling_by` and `extractions`. Contains all other fields
  sampler.validated_data =&gt; same problem as above
  </code></pre>
  
  <p>Any help would be appreciated! thanks</p>