Add Content-Type header to slack request for mattermost compatibility #153
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Mattermost tries to be API-compatible with Slack, but does not (currently) support incoming webhooks in JSON format without the 'Content-Type': 'application/json' header. Given that the Slack API documentation (not to mention RFC 7231) also specifies a preference for content type to be made explicit, adding the header here seems like reasonable way to make the webhooks handler work for Mattermost while continuing to work for Slack.