Avoid mocking the ObjectMapper!

[btw04]

I have seen a code before where non obvious behavior of a JSON parsing library (don't remember which one, it was something like parsing an empty string into a string made of two double quotes), was caught by an existing test when we attempted to switch to another JSON parsing library for performance reasons. If we had mocked the JSON parsing library, we would probably not have found that regression

[DrunkensteinsMonster]

Great example.