Fix #1941 - real fix for the workaround

[tchaloupka]

This bothered me so long that I've finally looked into it and found the issue. :)

Problem was that test for isSafeSerializer is using writeValue and readValue for checking and to pass correctly it requires both to compile.
In unittests, TestSerializer is used and works both as a serializer and a deserializer, so the test passes ok.
But that is not the case for ie JsonStringSerializer which has only one of those based on the provided range type.

[s-ludwig]

Ah, good catch! I'm wondering whether the two cases should be ANDed together to not miss an unsafe deserializer part when a safe serializer part is detected. Or maybe the best approach would be to just split this up into a isSafeSerializer and isSafeDeserializer?

[tchaloupka]

Right, splitting it up is probably the best. I've made the change.

[s-ludwig]

Thanks! Setting to auto-merge.