Add to collection: add integration test for custom number of stamps

[0pdd]

The puzzle 477-f35c9a3b from #477 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/controller/dto/AddToCollectionForm.java

Line 32 in 4cb3692

// @todo #477 Add to collection: add integration test for custom number of stamps

The puzzle was created by Slava Semushin on 13-Jun-18.