Fixes Slack message class

Mayra Cabrerarequested to merge
fixes-slack-message-class into master

What does this MR do?

Fixes Slack::Message class:

  • Parameters need to be sent along with the json argument
  • Response doesn't respond to success? but to code instead

Related to https://sentry.gitlab.net/gitlab/release-tools/issues/2416145/?referrer=slack

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