feat: enhanced type exception handling (#762)

Author: misantron

composer.json

@@ -34,6 +34,7 @@
     "psr-4": {
       "SendGrid\\Contacts\\": "lib/contacts/",
       "SendGrid\\EventWebhook\\": "lib/eventwebhook/",
+      "SendGrid\\Helper\\": "lib/helper/",
       "SendGrid\\Mail\\": "lib/mail/",
       "SendGrid\\Stats\\": "lib/stats/"
     },

lib/eventwebhook/EventWebhookHeader.php

@@ -8,7 +8,7 @@
  *
  * @package SendGrid\EventWebhook
  */
-abstract class EventWebhookHeader
+final class EventWebhookHeader
 {
     const SIGNATURE = "X-Twilio-Email-Event-Webhook-Signature";
     const TIMESTAMP = "X-Twilio-Email-Event-Webhook-Timestamp";