title: GAE Datastore date is not JSON serializable
body: <p>Im trying to send a the 10 latest post from an datastore in app engine. </p>
  
  <p>This is the database:</p>
  
  <pre><code>class Author(ndb.Model):
      """Sub model for representing an author."""
      identity = ndb.StringProperty(indexed=False)
      email = ndb.StringProperty(indexed=True)
  
  class Message(ndb.Model):
      """A main model for representing an individual Message entry."""
      author = ndb.StructuredProperty(Author)
      content = ndb.StringProperty(indexed=False)
      date = ndb.DateTimeProperty(auto_now_add=True)
  </code></pre>
  
  <p>And this is the code:</p>
  
  <pre><code>class Query(webapp2.RequestHandler):
      def post(self):
          message_name = self.request.get('db_name',
                                           DEFAULT_KEY)
          message_query = Message.query(ancestor=db_key(
                                        message_name)).order(-Message.date)
          messages = message_query.fetch(10)
  
          items = [] 
  
          for message in message_query:
              items.append({'id': message.author.identity, 
                            'email':message.author.email,
                            'content':message.content,
                            'date':message.date})    
  
  
          self.response.headers['Content-Type'] = 'application/json'   
          self.response.out.write(json.dumps(items))
  </code></pre>
  
  <p>And the error-message:</p>
  
  <pre><code>TypeError: datetime.datetime(2015, 3, 21, 15, 43, 58, 248650) is not JSON serializable
  </code></pre>
  
  <p>Why cant JSON return the date on that format?
  And how do i re-format it to the correct JSON format?</p>
  
  <p>Br</p>