authenticate method
Implementation
Future<POSUser> authenticate(String username, String password) async {
final map =
await HttpHelper.authenticate(username, password, domain, UserType.POS);
final name = map[User.dbName];
final surname = map[User.dbSurname];
final email = map[User.dbEmail];
final merchants = map['merchants'] != null
? List<Merchant>.from(
map['merchants'].map<Merchant>((m) => Merchant.fromMap(m)))
: <Merchant>[];
return POSUser(
merchants: merchants, name: name, surname: surname, email: email);
}