fix doc block get_request_header()
This method does not return FALSE as said in doc block, the correct
return is STRING or NULL.
diff --git a/system/core/Input.php b/system/core/Input.php
index 544b7c0..ada9fc6 100644
--- a/system/core/Input.php
+++ b/system/core/Input.php
@@ -766,7 +766,7 @@
*
* @param string $index Header name
* @param bool $xss_clean Whether to apply XSS filtering
- * @return string|bool The requested header on success or FALSE on failure
+ * @return string|null The requested header on success or NULL on failure
*/
public function get_request_header($index, $xss_clean = FALSE)
{