code: func (s *IssuesService) Lock(ctx context.Context, owner string, repo string, number int, opt *LockIssueOptions) (*Response, error) {
  	u := fmt.Sprintf("repos/%v/%v/issues/%d/lock", owner, repo, number)
  	req, err := s.client.NewRequest("PUT", u, opt)
  	if err != nil {
  		return nil, err
  	}
  
  	if opt != nil {
  		req.Header.Set("Accept", mediaTypeLockReasonPreview)
  	}
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // Lock an issue's conversation.
  //
  // GitHub API docs: https://developer.github.com/v3/issues/#lock-an-issue