title: AIrflow - task_instance.try_number is not working
body: <p>I am trying to send parameter to an airflow task in order to identify the last execution.</p>
  
  <p>The following code <strong>always</strong> send {"try_number": "1"} as POST data.</p>
  
  <p>Airflow version: 1.10.2</p>
  
  <p>Thanks</p>
  
  <pre><code>    xxx = SimpleHttpOperator(
          task_id='XXX',
          endpoint='/backend/XXX',
          http_conn_id='backend_url',
          data=json.dumps({"try_number": "{{ti.try_number}}"}),
          headers={"Content-Type": "application/json"},
          response_check=lambda response: response.json().get('status') == 'ok',
          dag=dag,
      )
  
  </code></pre>