Add missing email content (#589)

Without this line the code does not work.

Author: akerekes

appengine/mail/src/main/java/com/example/appengine/mail/MailServlet.java

@@ -68,6 +68,7 @@ private void sendSimpleMail() {
       msg.addRecipient(Message.RecipientType.TO,
                        new InternetAddress("[email protected]", "Mr. User"));
       msg.setSubject("Your Example.com account has been activated");
+      msg.setText("This is a test");
       Transport.send(msg);
     } catch (AddressException e) {
       // ...