Make schema generation aware of registered custom type

As in example with [documentation](https://github.com/orientechnologies/orientdb/wiki/Object-2-Record-Java-Binding#custom-types) (SecurityRole>String), it would crack in a in a moment you try to use auto-generated schema (`db.setAutomaticSchemaGeneration(true)`). Also, currently, for reasons above, you cannot created indexes against that field either.