SeriesServiceImpl.markAsSimilar(): add unit tests

The puzzle `1280-e234a7b0` from #1280 has to be resolved: 

https://github.com/php-coder/mystamps/blob/139115290d785550f58866d6add413968b49efb9/src/main/java/ru/mystamps/web/feature/series/SeriesServiceImpl.java#L304-L304

The puzzle was created by Slava Semushin on 12-Apr-20.