code: function(fullUrl, body, key, signature) {
        var data = fullUrl + sortPostParameters(body);
        var expected = getHash(data, key);
        return expected === signature;
      }
docs: Generate the signature from the full url and sorted post data to compart to the one provided.
  When mandrill is testing the existence of a url it can send a post request with no events and key set to 'test-webhook'.
  @param { string } fullUrl - domain and url of request - should match exactly the url specified in the webhook
  @param { string } body - request body
  @param { string } key - Mandrill Inbound Webhook auth key
  @param { string } signature - signature provided in the 'x-mandrill-signature' header
  @returns { boolean } True is signature is valid otherwise false