code: func (s *IssuesService) IsAssignee(ctx context.Context, owner, repo, user string) (bool, *Response, error) {
  	u := fmt.Sprintf("repos/%v/%v/assignees/%v", owner, repo, user)
  	req, err := s.client.NewRequest("GET", u, nil)
  	if err != nil {
  		return false, nil, err
  	}
  	resp, err := s.client.Do(ctx, req, nil)
  	assignee, err := parseBoolResponse(err)
  	return assignee, resp, err
  }
docs: // IsAssignee checks if a user is an assignee for the specified repository.
  //
  // GitHub API docs: https://developer.github.com/v3/issues/assignees/#check-assignee