title: Django serializer is_valid() returns True incorrectly
body: <p>I'm having a problem where my serializer.is_valid() method is returning true even though the provided data in the serialization is incomplete. I have a simple model that looks like this:</p>
  
  <pre><code>class Dummy(models.Model):
      first_name = models.TextField(max_length=128)
      last_name = models.TextField(max_length=128)
      organization = models.TextField(max_length=128)
      email = models.EmailField()
      can_contact = models.BooleanField()
  </code></pre>
  
  <p>for which I have created the following serializer:</p>
  
  <pre><code>class DummySerializer(serializers.ModelSerializer):
      class Meta:
          model = Dummy
          fields = ('first_name', 'last_name', 'organization', 'email',
                  'can_contact',)
  </code></pre>
  
  <p>If I then run this test (note the lack of <code>can_contact</code> in the input serialization):</p>
  
  <pre><code>&gt;&gt;&gt; serializer = DummySerializer(data = {'first_name': 'foo', 'last_name': 'bar', 'organization': 'quux inc', 'email': 'foo@example.com'})
  &gt;&gt;&gt; serializer.is_valid()
  True
  </code></pre>
  
  <p>If I were to attempt to save the serializer, I would get a constraint failure:</p>
  
  <pre><code>django.db.utils.IntegrityError: NOT NULL constraint failed: demo_dummy.can_contact
  </code></pre>
  
  <p>It appears specifically to be the BooleanField; any of the other fields will be correctly recognized as missing. What is going wrong with the serializer, and what do I do about it, short of doing my own manual input validation?</p>
  
  <p>Thanks for your time!</p>