ktor-client add toString() for generic params

[Rattenkrieg]

ktor-client doesn't support building form request from arbitrary parameters: it will happily consume values of predefined types such as String, Number etc but will fail in runtime (!) at a given line in case of say List<T> or Instant.
https://github.com/ktorio/ktor/blob/fb161cd7d268f2c0f62982d82d9264ca920173ad/ktor-client/ktor-client-core/common/src/io/ktor/client/request/forms/formDsl.kt#L63

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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)

[4brunu]

Looks good to me 👍