title: Slack user id is not recognized with error: 'user_not_found'
body: <p>I am doing a fetch to the slack.com/api/users.info method using:
  <code>return await fetch('https://slack.com/api/users.info?token=' + process.env.SLACK_AUTH_TOKEN + '&amp;user=' + slackUserId + '&amp;pretty=1', { method: 'POST' })</code></p>
  
  <p>The slack user id is correct and shows the same as I can see when clicking my profile in the Slack app, yet the Slack API can not find my user, as I get error: { ok: false, error: 'user_not_found' }. </p>
  
  <p>This is the first time I have this problem, it worked before, but I did not do any changes in the code.</p>
  
  <pre><code>{ token: 'xxx',
    team_id: 'xxx',
    api_app_id: 'xxx',
    event:
     { client_msg_id: 'xxx',
       type: 'message',
       text: 'test',
       user: 'UE8DHTXXX', // User id here
       ts: 'xxx',
       channel: 'DK037CXXX',
       event_ts: 'xxx',
       channel_type: 'im' },
    type: 'event_callback',
    event_id: 'xxxxxxxxx',
    event_time: 1559808987,
    authed_users: [ 'xxxxxxx' ] }
  
  Error:  { ok: false, error: 'user_not_found' }
  </code></pre>
  
  <p><img src="https://i.imgur.com/VZcWhxA.png" alt="Slack Id"></p>