Improve Head detaching code coverage

**[coveralls.io](https://coveralls.io/builds/2887964/source?filename=%2FLibGit2Sharp%2FCore%2FProxy.cs#L2438)** results show that, surprisingly, there's no test leveraging `git_repository_set_head_detached()`.

Is this a defect of some logic in the above layers that prevent this code path from being used or do we simply lack some test coverage?
