Keep extended route types in RouteType deserializer

[Naturseptime]

When reading GTFS data many of the extended route types are converted to old route types (e.g. 109 Suburban Railway -> 2 Rail). This leads to a loss of relevant information which cannot be recovered by the library user afterwards. I suggest to always keep the extended route type in the Route struct and add an extra function like old_route_type for compatibility.