title: React.js - default prop is not used with `null` is passed
body: <p>I have default props in my React component:</p>
  
  <pre><code>PropertyTitleLabel.defaultProps = {
      bedrooms: 1,
      propertyType: 'flat'
  };
  PropertyTitleLabel.propTypes = {
      bedrooms: PropTypes.number,
      propertyType: PropTypes.string
  };
  </code></pre>
  
  <p>But when I'm passing <code>null</code> to <code>bedrooms</code> like:</p>
  
  <pre><code>const bedrooms = null; // in real world API returns `null`
  &lt;Component bedrooms={bedrooms} /&gt;
  </code></pre>
  
  <p>It's not replaced with default prop :( Any ideas?</p>