create method
Implementation
Future<Message> create(String server, MessageCreateOptions options) async {
final url = Uri.parse('${baseUrl}api/messages');
final params = {'server': server};
final response = await client.post(url.replace(queryParameters: params), body: jsonEncode(options));
if (response.statusCode != 200) {
throw MailosaurError(response);
}
return Message.fromJson(jsonDecode(response.body));
}