#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.
I am not convinced by that. Values could be whatever, including weird `toString()` implementations. I would hate having the root cause of the issue hidden by having an exception when converting the value to its `String` representation.