Update the schema to include account_creation_date

As stated in https://developer.paypal.com/webapps/developer/docs/integration/direct/identity/attributes/ the Account creation date is one of the possible attributes readable for a user.

Right now this field is not include in the schema and consequently when getting the Userinfo for an account:

```
undefined method `account_creation_date=' for #<PayPal::SDK::Core::OpenIDConnect::DataTypes::Userinfo:0x7f1bc34fbc90>
```

I've not included any test, since the whole schema is untested now, previous updates to the schema have not included test and, finally, I don't know how you want to approach it.