IMAP: Declare IMAPConnection class as final (stub+test)

Updates the `IMAPConnection` class stub to make sure it has the `final` flag, and adds a test to verify it.

ext/imap/php_imap.stub.php

@@ -1,7 +1,7 @@
 <?php
 
 /** @generate-function-entries */
-class IMAPConnection {
+final class IMAPConnection {
 
 }
 

ext/imap/tests/imap_final.phpt

@@ -0,0 +1,11 @@
+--TEST--
+Check that IMAPConnection is declared final
+--SKIPIF--
+<?php
+extension_loaded('imap') or die('skip imap extension not available in this build');
+--FILE--
+<?php
+
+class T extends IMAPConnection {}
+--EXPECTF--
+Fatal error: Class T may not inherit from final class (IMAPConnection) in %s on line %d