title: Should JSON include null values
body: <p>I'm creating an API that returns results as JSON. Is there a current best practice for whether we should include keys in the result when the value is null?  For example:</p>
  
  <pre class="lang-js prettyprint-override"><code>{
      "title":"Foo Bar",
      "author":"Joe Blow",
      "isbn":null
  }
  </code></pre>
  
  <p>or</p>
  
  <pre class="lang-js prettyprint-override"><code>{
      "title":"Foo Bar",
      "author":"Joe Blow"
  }
  </code></pre>
  
  <p>Since the second is smaller I am leaning towards this style, but I'm not sure if there is a preferred style or not. From a client perspective it seems like both styles would be functionally equivalent. Any pros or cons to each?</p>