session: Fix for #18634, Warning on pipe character in session variable key

Changes based on feedback. Updated error message, added explicit test for session_write_close() and tidied up tests.

Author: mintopia

ext/session/tests/gh18634.phpt

@@ -1,16 +1,13 @@
 --TEST--
 GH-18634 (Using pipe character in session variable key causes session data to be removed)
---INI--
 --EXTENSIONS--
 session
 --SKIPIF--
 <?php include('skipif.inc'); ?>
 --FILE--
 <?php
-ob_start();
 session_start();
 $_SESSION['foo|bar'] = 'value';
-ob_end_clean();
 ?>
---EXPECTF--
-Warning: PHP Request Shutdown: Failed to write session data. Data contains invalid key "foo|bar". in Unknown on line 0
+--EXPECT--
+Warning: PHP Request Shutdown: Failed to write session data. Data contains invalid key "foo|bar" in Unknown on line 0

ext/session/tests/gh18634_2.phpt

@@ -0,0 +1,14 @@
+--TEST--
+GH-18634 (Using pipe character in session variable key causes session data to be removed - explicit session write)
+--EXTENSIONS--
+session
+--SKIPIF--
+<?php include('skipif.inc'); ?>
+--FILE--
+<?php
+session_start();
+$_SESSION['foo|bar'] = 'value';
+session_write_close()
+?>
+--EXPECTF--
+Warning: session_write_close(): Failed to write session data. Data contains invalid key "foo|bar" in %s on line %d