title: Problem to pass `http_conn_id` to SlackWebhookOperator
body: <p>I try create a <code>SlackWebhookOperator</code>, and using may HTTP connection, but he still traing use the <code>http_default</code>.</p>
  <pre class="lang-py prettyprint-override"><code>failed_alert = SlackWebhookOperator(
          task_id='slack_test',
          http_conn_id='slack_conn',
          webhook_token=slack_webhook_token,
          message=slack_msg,
          username='airflow')
  
  failed_alert.execute(context=context)
  </code></pre>
  <blockquote>
  <p>[2019-07-21 13:14:57,415] {{<strong>init</strong>.py:1625}} ERROR - Failed at executing callback</p>
  <p>[2019-07-21 13:14:57,415] {{<strong>init</strong>.py:1626}} ERROR - The conn_id <code>http_default</code> isn't defined</p>
  </blockquote>