ImageServiceImpl.replace(): add unit tests

The puzzle `1303-5b557695` from #1303 has to be resolved: 

https://github.com/php-coder/mystamps/blob/4c2104ad7a0860e73df660f3e32973e27cda6bd6/src/main/java/ru/mystamps/web/feature/image/ImageServiceImpl.java#L83-L83

The puzzle was created by Slava Semushin on 28-Apr-20.
