Use default match case for discriminator map

[cristiborta]

We overlooked the generic case when mapping keys from discriminator.

implicit lazy val decoder: Decoder[KeyBaseDiscriminator] = new Decoder[KeyBaseDiscriminator] {
  override def apply(c: HCursor): Result[KeyBaseDiscriminator] = c.value.asObject.toRight(DecodingFailure("Expected object", c.history)).flatMap { obj =>
    obj match {
      case _ if obj.contains("prefix") =>
        KeyBasePrefixString.decoder.tryDecode(c).fold(_ => KeyBasePrefixInt.decoder.tryDecode(c), Right(_))
      case _ if obj.contains("wildcard") =>
        KeyBaseWildcard.decoder.tryDecode(c)
     //we also need a generic case
      case other =>
        Left(DecodingFailure(s"unknown discriminator: ${other.keys.headOption.getOrElse("unknown_value")}", c.history))
    }
  }
}