Add Content-Type header to Slack notifier - req. for discord

[cdadityang]

I was working on building discord webhook for this repo, then I came across Discord - Slack Compatible Docs.

This means we can use slack notifier with Discord webhook(plus adding /slack at the end of discord webhook url). Ex:

config.add_notifier :slack, {
    webhook_url: "https://discord.com/api/webhooks/XXXX/XXXXXXXXX/slack"
  }

But the problem is, Discord webhooks requires header Content-Type: application/json set to it. This is pretty standard and recommended for all API apps. Seems that, for Slack API this header is optional, so right now the above discord webhook will fail.

So, I've just added the Content-Type header to the Slack Notifier, this will work for both Slack and Discord.

[coveralls]

Coverage increased (+0.003%) to 96.865% when pulling 1efde75 on cdadityang:master into e9f09b8 on smartinez87:master.