def subscribe(*args)
      params = arguments(args, required: [:topic, :callback]).params
      _merge_action!("subscribe", arguments.topic, arguments.callback, params)
      params['headers'] = HEADERS

      post_request("/hub", params)
    end
def subscribe(*args)
      params = arguments(args, required: [:topic, :callback]).params
      _merge_action!("subscribe", arguments.topic, arguments.callback, params)
      params['headers'] = HEADERS

      post_request("/hub", params)
    end
Subscribe to existing topic/event through pubsubhubbub

 @param [Hash] params
 @input params [String] :topic
   Required string. The URI of the GitHub repository to subscribe to.
   The path must be in the format of /:user/:repo/events/:event.
 @input params [String] :callback
   Required string - The URI to receive the updates to the topic.

 @example
  github = Github.new oauth_token: 'token'
  github.repos.pubsubhubbub.subscribe
    'https://github.com/:user/:repo/events/push',
    'github://Email?address=peter-murach@gmail.com',
    verify: 'sync',
    secret: '...'

 @api public