title: Django Rest Framework: Validation not working when setting a "validator" inside the model field
body: <p>I created a simple model for Tasks, the "days" field is supposed to be a list of integers from 0 to 6 depending on the days of the week that the tasks is supposed to be done. For example Monday, Wednesday and Friday would be 1,3,5</p>
  
  <pre><code>class Task(models.Model):
      user = models.ForeignKey(User, on_delete=models.CASCADE)
      name = models.CharField(max_length=255)
      time = models.TimeField()
      days = models.CharField(validators=[int_list_validator], max_length=16, blank=True)
  </code></pre>
  
  <p>The serializer looks like this:</p>
  
  <pre><code>class TaskCreateSerializer(serializers.ModelSerializer):
      class Meta:
          model = Task
          fields = ['name', 'time', 'days']
  </code></pre>
  
  <p>And the View looks like this:</p>
  
  <pre><code>class TaskCreateAPI(CreateAPIView):
      serializer_class = TaskCreateSerializer
  
      def perform_create(self, serializer):
          serializer.save(user=self.request.user)
  </code></pre>
  
  <p>When I do a POST request with some invalid data in the "days" field it will not raise any errors:</p>
  
  <pre><code>{
    "name": "Laundry",
    "time":"22:30",
    "days":"not an integer list"
  }
  </code></pre>
  
  <p>How can I have the validation done for my API request? What is the point of adding the validation inside the model field if it's not being validated when I do a POST request?</p>