GitLab unauthenticated requests fail

GitHub unauthenticated requests probably are not supported, but they do work.

GitHub(GitHubToken(None)).get_repo('jayvdb/flake8-putty').issues
{<GitHubIssue ....

However the same on GitLab results in RuntimeError

GitLab(GitLabPrivateToken(None)).get_repo('coala/coala-utils').issues
...
RuntimeError: ('{"message":"401 Unauthorized"}', 401)