SeriesServiceImpl.addPrice(): add unit tests

The puzzle `1340-c6d2a382` from #1340 has to be resolved: 

https://github.com/php-coder/mystamps/blob/49498ab3216e820d2cdce7e0eb94b96d125d40a3/src/main/java/ru/mystamps/web/feature/series/SeriesServiceImpl.java#L162-L162

The puzzle was created by Slava Semushin on 05-Jun-20.
