Fixes #153 - Set default platform when using custom payload

[DhesiMeRollin]

Fixes #153

Platform doesn't seem to be passed in through the request when a custom payload is used in the default response section of Dialogflow Console.

Fix - Set platform to PLATFORM_UNSPECIFIED when using custom payload

https://dialogflow.com/docs/reference/api-v2/rest/Shared.Types/Platform

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