Insert fails when using with Braintree PHP installed and having const "Braintree" on the saved class

[RobIsHere]

Because of the is_subclass_of check in php-rql
https://github.com/danielmewes/php-rql/blob/master/rdb/DatumConverter.php#L37

saving something like this fails because the check triggers the autoloader that in turn reveals an unfortunate code in Braintree's lib.

class User {
    const PAYMENT_PROVIDER = 'Braintree'
}

[Geekimo]

I've submitted a PR about this one. #148