Can't provide headers for preview features

From the [collaborators API doc](https://developer.github.com/v3/repos/collaborators/#list-collaborators):

> Note: The Nested Teams API is currently available for developers to preview. See the blog post for full details. To access the API, you must provide a custom media type in the Accept header:
>
> application/vnd.github.hellcat-preview+json

I don't see a way to add a header like this in the current API of this library.

Start with the collaborators API documentation linked in the issue, then locate the library's collaborators entry point and inspect how request options or headers are currently handled. Done means callers can provide the custom Accept media type required for preview APIs, with behavior covered by the project's existing tests if available.

索引モデルが issue の本文から書いたものです。