Review Requests endpoints under preview

[bobeagan]

https://developer.github.com/v3/pulls/review_requests/

• Need to update the configure() method to set the Accept header
• Should check on why the all() method has a $params
  • Doesn't appear to be anything in the docs that can be set
• Need to figure out if a short-term, backwards-compatible change should be made to create() and remove()
  • By adding a $teamReviewers parameter and setting the default value of $reviewers to an empty array
  • How to handle different shapes for preview vs not preview?
• Ideally, we would just remove the ['reviewers' => $reviewers] and make it take a more generic $params
  • To protect from any future changes and to more closely follow the pattern of other endpoints
  • Putting the responsibility on the developer to implement the correct information

[acrobat]

I've created a PR to address most of these remarks, see #984. Only the last remark is not implemented as it is "ok" for now and otherwise can be addressed in a separate PR to handle all these cases.