from_webhook is wrong type

[jasom]

here from_webhook is set to true but per the mattermost V4 api from_webhook is of type String, so it should actually be set to "true". Yes this is stupid, but clients that parse the messages strictly (e.g. matterhorn, or anything else that uses the haskell mattermost api implementation), will throw an error when encountering the malformed message.

[harshilsharma63]

Thanks for posting this @jasom . I'll update it.