def post_message(params)
      self.class.post('/chat.postMessage', body: params.merge({token: @token})).tap do |response|
        raise "error posting message: #{response.fetch('error', 'unknown error')}" unless response['ok']
      end
    end
def post_message(params)
      self.class.post('/chat.postMessage', body: params.merge({token: @token})).tap do |response|
        raise "error posting message: #{response.fetch('error', 'unknown error')}" unless response['ok']
      end
    end
send message to one channel as a single post with params text, channel, as_user