#3073 (feature/session): Enable Memcached::OPT_BINARY_PROTOCOL

Otherwise Memcached::touch() doesn't work.
diff --git a/system/libraries/Session/drivers/Session_memcached_driver.php b/system/libraries/Session/drivers/Session_memcached_driver.php
index 2ee922f..6652add 100644
--- a/system/libraries/Session/drivers/Session_memcached_driver.php
+++ b/system/libraries/Session/drivers/Session_memcached_driver.php
@@ -86,6 +86,7 @@
 	public function open($save_path, $name)
 	{
 		$this->_memcached = new Memcached();
+		$this->_memcached->setOption(Memcached::OPT_BINARY_PROTOCOL, TRUE); // required for touch() usage
 		$server_list = array();
 		foreach ($this->_memcached->getServerList() as $server)
 		{