[dart-dio] Correctly serialize path parameters

Created by: szekelyisz

If the value of a parameter in the path is something that was camelized (enum values containing dashes for example), toString() will just return the string representation of the camelized value, not the original, which will fail validation on the server. Thus path parameters need to be properly serialized as well, this will map camelized values back to their original value as per OAS.

Attn: @jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

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