[Kotlin] ktor JSON serialization handling

[kalinjul]

#12966 introduced a bug (#14044) where ktor does not perform json serializing at all in requests.
The Content-Type header is simply not set so content negotiation does not step in.

#14045 might also be a solution for this, but it seems to be stalled for some reason (too many changes?), so i created this minimal PR to fix the issue.

This commit fixes the issue for kotlin (ktor) multiplatform and ktor-jvm. Tested with multiplatform.

@jimschubert: Could you review? CI should work, just some download timeouts...

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[wing328]

cc @jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06)

[wing328]

thanks for the fix, which has been merged.