Fix this use case: load->vars->('foobar', '')

Previously, only the other syntax was working: load->vars->(array('foobar' => ''))
diff --git a/system/core/Loader.php b/system/core/Loader.php
index 6e5b58b..d4e6323 100644
--- a/system/core/Loader.php
+++ b/system/core/Loader.php
@@ -459,7 +459,7 @@
 	 */
 	public function vars($vars = array(), $val = '')
 	{
-		if ($val !== '' && is_string($vars))
+		if (is_string($vars))
 		{
 			$vars = array($vars => $val);
 		}