Issue #3386185 by mglaman, c_archer: Cast subject to string before passing to SDK

src/Plugin/Mail/SendGridMail.php

@@ -269,7 +269,7 @@ class SendGridMail implements MailInterface, ContainerFactoryPluginInterface {
     }
 
     $sendgrid_message->addCategories($categories);
-    $personalization0->setSubject($message['subject']);
+    $personalization0->setSubject((string) $message['subject']);
     $from = new From($data['from'], $data['fromname']);
     unset($data);
     # Set the from address and add a name if it exists.