error when saving project

I tried to write a script to change a project's visibility level. After retrieving the project via 'all_projects()', I did:

```
p.visibility_level = 0  # Private
p.save()
```

However, this fails with

```
gitlab.GitlabCreateError: 400: {u'path': [u'has already been taken'], u'name': [u'has already been taken']}
```

What am I doing wrong?
