title: Django BooleanField accepting non-boolean type object
body: <p>I have a BooleanField on Question model:</p>
  
  <pre><code>class Question(models.model):
      is_deleted = BooleanField()
  </code></pre>
  
  <p>Why does it accept non-boolean type python object?</p>
  
  <pre><code>&gt;&gt;&gt;que_obj = Question.objects.create()
  &gt;&gt;&gt;que_obj.is_deleted='yes'
  &gt;&gt;&gt;que_obj.save()
  &gt;&gt;&gt;que_obj.is_deleted
  &gt;&gt;&gt;'yes'
  </code></pre>