This reverts commit 75e5464.

* mutant signalled some unhandled mutations around key.to_s

* which lead to the investigation that we should not need key.to_s at
  all

* the reason we have to do it is because of the tests,
  transformation tested on their own do not pass through
  SymbolizeMetadaKeys which is part of every mapper — when such
  transformation is not present as part of the pipeline the keys wound
  not be symbols: thus the key.to_s fallback

* we actually should check both variants, with SymbolizeMetadataKeys as
  in the mappers we provide and when transformation is used standalone
  — in case someone composes mappers as they wish

At this point I don't understand why ostruct would always have symbols,
while active_support_time_with_zone would differentiate.

WELP.