title: Django REST Framework's APIClient sends None as 'None'
body: <p>In my tests, I send mock data of models that I've passed through the serializer. The <code>serializer.data</code> looks something like this</p>
  
  <pre><code>{
      "field": None
  }
  </code></pre>
  
  <p>However, the data that my API receives is formatted like</p>
  
  <pre><code>{
      "field": "None"
  }
  </code></pre>
  
  <p>which is a problem because I'm trying to specify a foreign key that is allowed to be null. Shouldn't the <code>APIClient</code> convert None into null instead of unicode?</p>
  
  <p>Is there any way to fix this or get around it?</p>
  
  <p>Here's my serializer</p>
  
  <pre><code>class MyModelSerializer(serializers.ModelSerializer):
  
      field = serializers.PrimaryKeyRelatedField(
          queryset=OtherModel.objects.all(), required=False, allow_null=True)
  </code></pre>
  
  <p>And my create method in a viewset</p>
  
  <pre><code>def create(self, request):
      model = MyModel()
      serializer = MyModelSerializer(model, data=request.data)
      if serializer.is_valid():
          serializer.save(owner=request.user)
          return Response(serializer.data, status=201)
      return Response(serializer.errors, status=406)
  </code></pre>
  
  <p>Also my model class</p>
  
  <pre><code>class MyModel(models.Model):
      field= models.OneToOneField(
          OtherModel, blank=True, null=True)
  </code></pre>