SeriesInfoExtractorServiceImpl.extractSellerUrl(): add unit tests

[0pdd]

The puzzle 695-2a031cd8 from #695 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/service/SeriesInfoExtractorServiceImpl.java

Line 278 in 0081e7a

// @todo #695 SeriesInfoExtractorServiceImpl.extractSellerUrl(): add unit tests

The puzzle was created by Slava Semushin on 08-Feb-18.