message: extend slack_botkit.middleware.format middleware
  add blocks filed to platform message in case it is exist
diff: @@ -573,8 +573,10 @@ function Slackbot(configuration) {
  platform_message.link_names = message.link_names || null;
  platform_message.attachments = message.attachments ?
  JSON.stringify(message.attachments) : null;
  - platform_message.blocks = message.blocks ?
  - JSON.stringify(message.blocks) : null;
  + if(message.blocks){
  + platform_message.blocks = JSON.stringify(message.blocks);
  + }
  +
  platform_message.unfurl_links = typeof message.unfurl_links !== 'undefined' ? message.unfurl_links : null;
  platform_message.unfurl_media = typeof message.unfurl_media !== 'undefined' ? message.unfurl_media : null;
  platform_message.icon_url = message.icon_url || null;
  