Add integration tests

This is an ongoing work item to add integration tests for the entire library. Unlike the existing unit tests that spin up a local HTTP server that mimics certain behavior of the API, these integration tests will call against the live GitHub API. As such, they will likely take a lot longer to execute and will not be run near as often. I've set things up with aff0a13d884ff5dc09bccb6c1ce081a82213bfab and will continue working from there.

I'm not 100% settled yet on how extensive these tests need to be. For example, we don't need to test the value of every field in a response. The biggest goal is to have a test suite that can be run when things like the [default media type changes](https://developer.github.com/changes/2014-04-09-reminder-about-default-media-type-change/) to make sure the library is still working properly. I also have a separate test suite that I'm working on to identify fields in responses that we aren't mapping into our Go structs.

Commencez par la configuration des tests d’intégration du commit aff0a13d884ff5dc09bccb6c1ce081a82213bfab et comparez-la aux tests unitaires existants qui utilisent un serveur HTTP local. Définissez le périmètre nécessaire avant de poursuivre ; le résultat attendu doit être une suite de tests de l’API GitHub en conditions réelles qui détecte les problèmes de compatibilité, tels que les changements du type de média par défaut, sans vérifier chaque champ de réponse.

Rédigé par le modèle d'indexation à partir du texte de l'issue.