Reverted rui_string change.

This had knock-on effects as can be seen in #1306. Issue #122 has been
reopend until it is fixed properly.
diff --git a/system/core/URI.php b/system/core/URI.php
index 705575a..cf82c58 100755
--- a/system/core/URI.php
+++ b/system/core/URI.php
@@ -645,10 +645,10 @@
 	 */
 	public function ruri_string()
 	{
-		return implode('/', $this->rsegment_array());
+		return '/'.implode('/', $this->rsegment_array());
 	}
 
 }
 
 /* End of file URI.php */
-/* Location: ./system/core/URI.php */
\ No newline at end of file
+/* Location: ./system/core/URI.php */