Add support for draft pullrequests

[acrobat]

php-github-api/lib/Github/Api/PullRequest.php

Lines 148 to 165 in 325b315

	public function create($username, $repository, array $params)
	{
	// Two ways to create PR, using issue or title
	if (!isset($params['issue']) && !isset($params['title'])) {
	throw new MissingArgumentException(['issue', 'title']);
	}
	if (!isset($params['base'], $params['head'])) {
	throw new MissingArgumentException(['base', 'head']);
	}
	// If `issue` is not sent, then `body` must be sent
	if (!isset($params['issue']) && !isset($params['body'])) {
	throw new MissingArgumentException(['issue', 'body']);
	}
	return $this->post('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/pulls', $params);
	}

We should check the $params array if an option draft exists and is set to true. If so we should set the preview accept-header application/vnd.github.shadow-cat-preview+json

See: https://developer.github.com/changes/2019-02-14-draft-pull-requests/