Authenticates a Mandrill Webhook POST request.
The test doesn't contain the correct authentication data, if you want to test this yourself you'll need to generate your own data.
You can do this by going to requestb.in and creating a
private bin. Then copy the bin URL and use it to create a new webhook in Mandrill. The authKey is the key value for the webhook so you can copy that in. Next click
the test event button and refresh your requestbin. Finally copy the mandrill_events and X-Mandrill-Signature and run the test.