Add SendGrid X-SMTPAPI Headers

Mailing.php

--- CRM/Mailing/BAO/Mailing.php	2019-05-15 20:00:28.000000000 +0200
+++ CRM/Mailing/BAO/Mailing.php.new	2019-07-10 10:56:06.561926552 +0200
@@ -1034,6 +1034,14 @@
     if ($isForward) {
       $headers['Subject'] = "[Fwd:{$this->subject}]";
     }
+
+    // Add SendGrid X-SMTPAPI Headers
+    $json_string = ['category' => $this->name];
+    if ($event_queue_id) {
+      $json_string['unique_args'] = ['event_queue_id' => $event_queue_id];
+    }
+    $headers['x-smtpapi'] = json_encode($json_string);
+
     return [&$verp, &$urls, &$headers];
   }