Simplest possible patch to allow custom modules for json serialization/deserialization

Pretty hacky as it exposes the internals of ResponseBuilder and in particular couples Finatra API to Jackson API. 

Another less coupled approach (but for more code) would be to add a JsonMapping interface to Finatra and have a Jackson implementation. However then I would also either need to make jsonMapper a var in ResponseBuilder so it can be reset (yuk) or provide a constructor param on ResponseBuilder.