title: Serialize Java 8 LocalDate as yyyy-mm-dd with Gson
body: <p>I am using Java 8 and the latest <code>RELEASE</code> version (via Maven) of <a href="https://github.com/google/gson" rel="noreferrer">Gson</a>.
  If I serialize a <a href="https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html" rel="noreferrer"><code>LocalDate</code></a> I get something like this</p>
  
  <pre><code>"birthday": {
          "year": 1997,
          "month": 11,
          "day": 25
  }
  </code></pre>
  
  <p>where I would have preferred <code>"birthday": "1997-11-25"</code>. Does Gson also support the more concise format out-of-the-box, or do I have to implement a custom serializer for <code>LocalDate</code>s? </p>
  
  <p>(I've tried <code>gsonBuilder.setDateFormat(DateFormat.SHORT)</code>, but that does not seem to make a difference.)</p>