Handle unparsable dictionaries better

[mac-cain13]

Only [String: *] dicts are supported by JsonGen, since a Json dictionary key always is a string. I tried to outsmart JsonGen by creating this:

enum MyKey: String {
  case SomeKey = "SomeKey"
}

struct MyJsonStruct {
  let someDict: [MyKey: OtherJsonStruct]
}

JsonGen gave me the impression that it could work because the Dictionary.decodeJson method takes a key decoder that is somewhat generic. But it really only will work with String keys because of the direct case to [Key: Value] that JsonGen performs, maybe this is something that could be a little stricter.