Fixed int typecasting of limit parameter in DB_active_rec.php

If an empty string was passed, the typecast was changing it to 0. This
produced "LIMIT 0" which returned no results. This must be a bug and not
a feature because there are other params after limit in both
$this->db->limit and $this->db->get.
diff --git a/system/database/DB_active_rec.php b/system/database/DB_active_rec.php
index 841ede2..a16363c 100644
--- a/system/database/DB_active_rec.php
+++ b/system/database/DB_active_rec.php
@@ -874,7 +874,7 @@
 	 */
 	public function limit($value, $offset = '')
 	{
-		$this->ar_limit = (int) $value;
+		$this->ar_limit = $value;
 
 		if ($offset != '')
 		{