Better test cases for user API

In brief, the current test cases don't test anything except that the server is running and does respond to requests. Moreover, the API is inconsistent and incomplete.

I created a couple of test cases on the user API to illustrate a more consistent and complete approach using the user API as an example.
Server should respond with a representation of the newly created entity, i.e. the new user. Id is still included for the App to work with.