Spec tests demonstrating issue #29

This PR needs PR #30 to be merged (for basic junit pom.xml dependencies on restx-server-simple).

1 of the 3 provided spec tests are not working at the latest step : `GET /sessions/current` returns something after the session has been deleted, which sounds weird.

Please, have a look at [this branch](https://github.com/fcamblor/restx/tree/temp-demo-issue29) which hacks the restx-server-simple pom.xml in a way which should never be commited (that's why I dissociated this from this PR), in order to easily compare the restx-server-jetty and the restx-server-simple behaviours.


Note that I could have added the same tests in the restx-server-jetty project, but I can't ATM, because of circular dependencies (restx-spec-tests depends on restx-server-jetty, so we cannot let restx-server-jetty depend on restx-spect-tests)
I'll try to refactor a bit restx-spect-tests in order to make RestxSpecRule rely on a SPI (or maybe better, a @Component) to find its WebServerSupplier.
WDYT about this (before I start working on it) ?