Add an __UNDEFINED_ value to all ProtoEnums

@loganj 
Basically `Message` instances serialize with Gson almost perfectly without any help, this module just cleans up some of the internal fields that Wire uses that shouldn't appear in the JSON.

I use it for debugging sometimes and it can be useful to compare payloads between json and proto network endpoints. All I'm saying is that you shouldn't count on it to do the kind of forward/backwards compatibility tricks that you get with protos.