title: Serializer validated_data is empty even when is_valid is True
body: <p>Following code returning an empty json.</p>
  
  <pre><code>def create_account(request):
      serializer = UserSerializer(data={'name': request.GET.get('name'), 'surname': request.GET.get('surname'),
                                        'password': request.GET.get('password'), 'email': request.GET.get('email')})
      if not serializer.is_valid():
          return HttpResponse(status=401,content=serializer.errors)
  
      serializer.save()
      return Response(serializer.data)
  </code></pre>
  
  <p>And this is my serializer</p>
  
  <pre><code>class UserSerializer(serializers.ModelSerializer):
      class Meta:
          model = User
          fields = ('name', 'surname', 'password', 'email')
  
      def create(self, validated_data):
          return User.objects.create(**validated_data)
  </code></pre>
  
  <p>When i send a request, <code>serializer.initial_data</code> is filled correctly. <code>is_valid</code> is returning <code>True</code> but <code>validated_data</code> is empty. </p>