title: channel_not_found exception while sending the message through incoming webhook
body: <p>Am using incoming webhooks to send messages, however am overiding the channelID to send to particular channel(as mentioned in here:<a href="https://api.slack.com/incoming-webhooks" rel="noreferrer">https://api.slack.com/incoming-webhooks</a>) by using something like this</p>
  
  <pre><code>   {
  "channel": "#my_channel",
  "text": "This message will appear in #other-channel"
    } 
  </code></pre>
  
  <p>Am able to receive messages into slack when i give my channelID, but when i give someone else channelID(valid channel ID), am getting channel_not_found exception.</p>
  
  <p>Please let me know what might went wrong.</p>