added new test case to force rollback when table participating in transa

Hi!
First of all thanks for putting so much effort in Dexie.js. It really simplifies the usage of the IndexeDB API a lot.
While working on a project I noticed that the new test case in this pull request is not working as I would have expected. I.e. the transaction is processed fine, although the test adds an object to a table that is not listed in Dexie.transaction().