.retry() gets invoked even upon successful request

[IAMtheIAM]

BUG: The retry() method gets invoked on every request, even if the request is successful. See: https://github.com/visionmedia/superagent/blob/95acfc700a11ce2377d6bb524749d94e0978b846/src/client.js#L647-L650

However, it should only get invoked on an unsuccessful request. Currently we have to check for err to ensure that the request actually failed, or the logic will get executed there undesirably.

The code should be updated to:

  if (err !== null && this._shouldRetry(err, res)) {
    return this._retry();
  }

I will make a PR if this is agreed to.