Current library doesn't support this option.
https://developer.github.com/v3/repos/branches/#update-branch-protection
Working on PR to add it.
I think we can close this issue, as it's already supported. https://github.com/google/go-github/blob/master/github/repos.go#L760
Thank you, @sayboras! Closing.
Most helpful comment
I think we can close this issue, as it's already supported. https://github.com/google/go-github/blob/master/github/repos.go#L760