query: 2.0.0 - (RED) The keys of the top10-topic are deserialized as JSON

query: 2.0.0 - (RED) The keys of the top10-topic are deserialized as JSON

* The seemingly straightforward change leds to a very strange and
  inconsisten error-situation.
* Only the integration-test fails, while the topology-test works as
  originally expected.
* The cause of the error is a missing serde-config for the key of the
  ``rankings``-``KStream``, which defies easy explanation.
* The best explanation is, that the ``map()``-operation - despite
  possibly changing the type of the key and/or value - does not by itself
  define a parameter for specifing a corresponding serialization-config.
* The reason for this is, that the operation does not define the complete
  operation by itself.
* In order to take effect, it has to be combined with a second operation,
  that actually creates the outgoing topic.
* Without that second DSL-operation, `map()` simply would yield no action.
* And that is, why the serialization has to be defined on that second
  operation and cannot be defined on `map()` itself.
* But the really strange thing about the error is, that it _only_ shows up
  in `QueryApplicationIT`.
* It does not show in `QueryStreamProcessorTopologyTest` _and_ it does
  _not_ show up, if the application is compiled and started in the
  docker-setup.
* One possible explanation for this wired behaviour might be a bug or
  misconception in the interpretation of the beforehand build topology,
  that leads to a non-deterministic behaviour.
* Another possible explanation might be subtle differences in the internal
  caching behaviour -- but that seems unlikely, because tests, that are
  based on the `TopologyTestDriver` do not cache and are very (on the oposit)
  very handy if one wants to reveal bugs concerning the serialization and
  and running the application with the caching settings from the IT does
  not show the error.