Update series: add integration test

The puzzle `785-2f662661` from #785 has to be resolved: 

https://github.com/php-coder/mystamps/blob/7be4e62acf67e49399209dfe1cb5746759cab445/src/main/java/ru/mystamps/web/feature/series/RestSeriesController.java#L43-L43

The puzzle was created by Slava Semushin on 24-May-20.