title: Airflow Slack failed to send Message
body: <p>I am trying to achieve Airflow integration with Slack, 
  have received the webhook URL, and created the connection as below. Why is it showing google.com ??</p>
  
  <p>Why is it using the default http_conn_id and connecting to google ??</p>
  
  <p>But got an error as below</p>
  
  <p>ERROR - Error in sending a message to Slack channel #airflow-alerts
              by Airflow: 404:Not Found</p>
  
  <pre><code>{base_hook.py:83} INFO - Using connection to: id: http_default. Host: https://www.google.com/, Port: None, Schema: None, Login: None, Password: None, extra: {}
  {logging_mixin.py:95} INFO - [2020-05-29 12:43:21,374] {http_hook.py:128} INFO - Sending 'POST' to url: https://www.google.com//T00A6ASFHD8S/G1FDF4K/a3zfKsadfsrScxgadfsdafOIgIvgW
  
  requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://www.google.com//T00A6ASFHD8S/G1FDF4K/a3zfKsadfsrScxgadfsdafOIgIvgW
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/34MdW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/34MdW.png" alt="enter image description here"></a></p>
  
  <p>But I got the below error, unable to figure out </p>