title: Boto DynamoDB2 conditional put_item
body: <p>I am trying to have put_item to check if there is item with the same HashKey before actually adding the new item.</p>
  
  <p>According to boto DynamoDB2 document, it is possible to do it with "Conditional Put".</p>
  
  <p>I tried following command but no luck.</p>
  
  <pre><code>connection.put_item('table',item={'locationId':'a1', 'timestamp': time.time()}, expected={'locationID':False})
  </code></pre>
  
  <p>The error message is as following.</p>
  
  <pre><code>boto.exception.JSONResponseError: JSONResponseError: 400 Bad Request
  {u'Message': u'Expected null', u'__type': u'com.amazon.coral.service#SerializationException'}
  </code></pre>
  
  <p>Does anyone have a conditional put with DynamoDBv2? </p>
  
  <p>Thanks to all in advance.</p>