Fixed ntpath.expandvars to not replace references to non-existing
variables with nothing.  Also added tests.
This fixes bug #494589.
diff --git a/Misc/NEWS b/Misc/NEWS
index b889b09..3d742f5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -109,6 +109,8 @@
 Library
 -------
 
+- Bug #494589: make ntpath.expandvars behave according to its docstring.
+
 - Changed platform module API python_version_tuple() to actually
   return a tuple (it used to return a list)