Allow descent below root multiple times
Fixes http://bugzilla.ximian.com/show_bug.cgi?id=71053

Index: System/Uri.cs
===================================================================
--- System/Uri.cs	(revision 46893)
+++ System/Uri.cs	(working copy)
@@ -1064,9 +1064,8 @@
 
 				if (current == "..") {
 					if (result.Count == 0) {
-						if (i == 1) // see bug 52599
-							continue;
-						throw new Exception ("Invalid path.");
+						// see bugs 52599 and 71053
+						continue;
 					}
 
 					result.RemoveAt (result.Count - 1);