Make test_mhlib run again.

There's some wierdness here, but the test ran before and not after,
so I'm just hacking the change out.  Someone more motivated than
me can work out what's really happening.

Raymond: *PLEASE* run the test suite before checking things like
this in!

Lib/mhlib.py

@@ -788,7 +788,7 @@ class IntSet:
         self.pairs = []
         self.sep = sep
         self.rng = rng
-        if data is not None: self.fromstring(data)
+        if data: self.fromstring(data)
 
     def reset(self):
         self.pairs = []