Get all protected branches of a repo

Hi,
the `listBranches()` method doesn't allow to filter protected branches.
Is there a workaround to get all protected branches of a repository?

Github API: https://developer.github.com/v3/repos/branches/#list-branches

Start with the listBranches() method and compare its available options with the linked GitHub repository branches API documentation. The work is complete when the wrapper provides a clear way to retrieve all protected branches for a repository, with behavior verified against the API response.

索引モデルが issue の本文から書いたものです。