#117 fix SPI tests by adding missing key/value type checks

Missing key/value type checks are fixed, but the hit rate test is still failing because hit rate is not yet implemented. I left that test as ignored and I opened #122 to track the missing implementation.
ClassCastException sure seems tempting here, at the lowest level - but I wonder if we should actually throw a `CacheAccessException` all the way up to the caller of the API, instead?