title: Slack API Using \n for new lines does not work in chat.postMessage (golang)
body: <p>I'm using the Slack web API to post messages to a channel in Go. I'm trying to support multi-line messages in the text field. According to the documentation simply adding a \n should work but it's not working. When posting the \n appears in the text and there's no line break. </p>
  
  <p>Here's the code I'm using:</p>
  
  <pre><code>func PostMessage(token, channelName, userName, text string) error {
      uv := url.Values{}
      uv.Add("token", token)
      uv.Add("channel", channelName)
      uv.Add("username", userName)
      uv.Add("text", text)
  
      resp, err := http.PostForm("https://slack.com/api/chat.postMessage", uv)
      if err != nil {
          return err
      }
      return nil
  }
  
  func main() {
      if err := PostMessage("xxxx", "#test-channel", "API", "This should be the first line\nThis should be the second line"); err != nil {
          panic(err)
      }
  }
  </code></pre>