Using NewAdapter will reset all tables already registered in default

[YuPengZhangL]

Problematic code:

func (a *Adapter) registerModel() {
	one.Do(func() {
		orm.ResetModelCache()
		orm.RegisterModel(&CasbinRule{tableName: a.tableName, tablePrefix: a.tablePrefix})
	})
}

Phenomenon:
orm.ResetModelCache() will clear all the existing default table structures registered in the ORM. Hope to improve this."