Fix a CI_Input::set_cookie() bug

Found on StackOverflow: https://stackoverflow.com/questions/41925028/codeigniter-config-overrides-set-cookie-parameters
diff --git a/system/core/Input.php b/system/core/Input.php
index d7cd292..af4f87c 100644
--- a/system/core/Input.php
+++ b/system/core/Input.php
@@ -359,7 +359,7 @@
 	 * @param	bool		$httponly	Whether to only makes the cookie accessible via HTTP (no javascript)
 	 * @return	void
 	 */
-	public function set_cookie($name, $value = '', $expire = '', $domain = '', $path = '/', $prefix = '', $secure = FALSE, $httponly = FALSE)
+	public function set_cookie($name, $value = '', $expire = '', $domain = '', $path = '/', $prefix = '', $secure = NULL, $httponly = NULL)
 	{
 		if (is_array($name))
 		{
@@ -388,15 +388,13 @@
 			$path = config_item('cookie_path');
 		}
 
-		if ($secure === FALSE && config_item('cookie_secure') === TRUE)
-		{
-			$secure = config_item('cookie_secure');
-		}
+		$secure = ($secure === NULL && config_item('cookie_secure') !== NULL)
+			? (bool) config_item('cookie_secure')
+			: (bool) $secure;
 
-		if ($httponly === FALSE && config_item('cookie_httponly') !== FALSE)
-		{
-			$httponly = config_item('cookie_httponly');
-		}
+		$httponly = ($httponly === NULL && config_item('cookie_httponly') !== NULL)
+			? (bool) config_item('cookie_httponly')
+			: (bool) $httponly;
 
 		if ( ! is_numeric($expire))
 		{