Extract commonly used data

The `SomeUsers` and `SomePapers` can provide a bunch of users and papers
that can be used by any scenario. This avoids the need of recreating
such data in several scenarios
Fix annoying recurring bug

headers must be written before response body. For example, setting the
status code after having written the response body will issue an error.
By making the `writeJson` method return `Unit` we cannot chain the calls
after this one, which avoid stupid bugs that were to be found at several
places in existing code.
Add some user data scenarios