Add Unmangled Discriminator Property Name to Discriminator Class

[da1910]

The Discriminator class contains some information to inform how deserialization should occur, given the property name and the mapping to subclasses I would expect to be able to dispatch appropriately.

Unfortunately the propertyName property is mangled before it is passed to the template generation, so for example in the python client generator, a property of petSpecies is only available as pet_species. If you then want to use this property to fetch data from the API response, you need to guess what the original form of the property name was.

It would be very helpful if the Discriminator class also stored the original, unmangled, property name. This would make the class actually useful for the purpose of dispatching to subclasses whilst deserializing responses.