Tests

This PR groups [PR#19](https://github.com/heroku/heroku-bouncer/pull/19) and [PR#20](https://github.com/heroku/heroku-bouncer/pull/20) and includes tests for different endpoints and configuration options, addressing #6.

The tests are testing the behavior (authentications/redirections, `rack.env[bouncer.*]`) but could be improved by testing how the middleware affects (or doesn't) the browsing session.