Initial specs and slight refactoring for Minitel

@brandur @pedro what do you think?
Another approach to get the same confidence perhaps would be if I could get the requests that happened in an array or something. Then I could use regular assertions on them. 

I saw this with the Mail gem where it has [a .deliveries](https://github.com/mikel/mail#using-mail-with-testing-or-specing-libraries) part during tests.