title: Bot postMessage missing icon and name
body: <p>Using a custom bot, I can <code>postMessage</code>s (<a href="https://api.slack.com/methods/chat.postMessage" rel="noreferrer">doc</a>) into a channel (with the bot's auth-token). However, the messages don't show my bot's icon or name, just the default bot's one:</p>
  
  <p><a href="https://i.stack.imgur.com/F7FMU.png" rel="noreferrer"><img src="https://i.stack.imgur.com/F7FMU.png" alt="slack-bot"></a></p>
  
  <p>When I change topic, it works fine (i.e. the icon and name of my custom bot appear).</p>
  
  <p>Is there some extra configuration needed?</p>