title: How can I set default value in serializers?
body: <p>I have these serializers:</p>
  
  <pre><code>&gt;---id = serializers.IntegerField()
  &gt;---name = serializers.CharField()
  &gt;---age = serializers.IntegerField()  
  </code></pre>
  
  <p>Now, I give the serializers this data:</p>
  
  <pre><code>{'id': 1, 'name': 'cc'}
  </code></pre>
  
  <p>I don't give a value for <code>age</code>.
  how can I set a default value to <code>age</code> in the serializers?</p>
  
  <p>I want to get this, where the <code>12</code> is a default value:</p>
  
  <pre><code>{'id': 1, 'name': 'cc', 'age': 12}
  </code></pre>