columnsOf method
toColumns without the type argument.
Dart checks generic function values at runtime, so tearing off
toColumns from an MssqlTableBinding<Object?> fails when the binding is
really an MssqlTableBinding<CustomerRow>. The relation loader works
across table types, so it goes through this instead.
Implementation
Map<String, Object?> columnsOf(Object? row) => toColumns(row as TRow);