test message for custom webhook only works on plain text

[zhongnansu]

Describe the bug no matter what content type that user defines when creating the custom webhook channel. For sending test message, it's always send a plain text "test mesaage xxx xxx". Because it's pre-defined plain text in backend for any channel type. That leads to failure, since custom webhook could be created to accept any content type.

e.g. Test message will never be sent successfully to a custom webhook defined as accepting json. Furthermore, accepting json could be defined to accept any json or json with certain fields with certiain keys

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