Pagination for preview endpoints

I have some code like below but it appears to be dropping the `Accept` header for the preview of projects endpoint when trying to paginate. Is there some other way that I should be running configure to make that work?

```
$cardsApi = $githubClient->api('repo')->projects()->columns()->cards()->configure();
$paginator = new Github\ResultPager($githubClient);
$parameters = [$columnId];
$cards = $paginator->fetchAll($cardsApi, 'all', $parameters);

Start by reading the repo/projects/columns/cards configure chain and Github\ResultPager::fetchAll used in the example, focusing on how preview headers are carried into paginated requests. Reproduce the pagination call and verify that the projects preview Accept header is preserved for every request.

索引モデルが issue の本文から書いたものです。