More Testing

In order to give this some love, here's a small pr with a test. 
Change in the DoctrineDatabase::getVatRateData is to apply the "return asap principle". 

Some tasks to do here (as far i see)
- [x] Vat Doctrine Database test
- [ ] Vat Handler
- [ ] Content Vat Database test
- [ ] Content Vat Handler

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Further more, we could add more info in the README file. Thoughts? More test needing?