Unhandled promise rejection when using Repository.isCollaborator()

When calling Repository.isCollaborator(), the a _UnhandledPromiseRejectionWarning: Error: 404 error making request get https://api.github.com/repos/org/repo/collaborators/xxx: "Not Found"_ is thrown when a user is not a collaborator, instead of returning false, and if the user is a collaborator, the request object is returned with blank data field. Based on the documentation, the promise should resolve to a boolean, but this doesn't seem to be the case with how actual behaviour.

Start at the Repository.isCollaborator() entry point and reproduce both cases described against the GitHub collaborators endpoint: a non-collaborator and a collaborator. Compare the observed promise result with the documented boolean behavior; done means both cases resolve to the expected boolean without an unhandled rejection.

Written by the indexing model from the issue text.