This is an example of a fix for loggly/winston-loggly-bulk#12 with a small amount of code change.

Note that these lines:
```
    if (timerFunction === null) {
      timerFunction = setInterval(function () {
        sendBulkLogs();
      },30000);
    }
```

produce the same bug when `isBulk` is `true`. See loggly/winston-loggly-bulk#13