Testing response on requesting non-existent resource -> Status is 200 instead of expected 404
Implemented ResourceNotFoundException using the @ResponseStatus annotation to tell Springs ResponseStatusExceptionResolver(active by default) which HttpStatus to return when this Exception is thrown ->  Test now passes.
Refactored UserControllerTest - test methods are ordered by Http verbs: GET, PUT, POST, DELETE
Testing behavior of PUT requests to entire collection without explicit RequestMethod declaration -> Status is 200, performed operation is findAll() :(
! With the RequestMethod back in place the response is 405 (MethodNotAllowed) which seems more comprehensive than the recommended NotFound response. -> Should be discussed
Implemented PUT request test for an invalid userId -> Status 200 instead of 404
Implemented specific Exceptions to allow the controller to react correctly
Fixed buggy methods (updateUser did not use the userId!), used exceptions to react on the event of a non existing user
Tried to get a NoContent response on PUT request -> Status is 400 (BadRequest) instead
-- Modified Test for evaluating response in debugger
-- Internal exception HttpMessageNotReadableException gets handled resulting in the BadRequest. Modified test again to take a closer look at the MvcResult.