title: Slack bot cannot post message to private channel
body: <p>I follow <a href="https://api.slack.com/methods/chat.postMessage" rel="noreferrer">Slack bot post message documents</a></p>
  <p>it works perfectly when post to public channel, but get error <code>[error] =&gt; channel_not_found </code> when post to private channel, I even change channel_name to channel ID (ex: <code>G1HCG4BQ9</code>)</p>
  <p>I already authorized with both <code>chat:write:user</code> and <code>chat:write:bot</code> permissions</p>