Re-enable a new empty-string test added during the NFS sprint,

but disabled then because str and unicode strings gave different
results.  The implementations were repaired later during the
sprint, but the new test remained disabled.

Lib/test/string_tests.py

@@ -494,12 +494,7 @@ class CommonTest(unittest.TestCase):
 
         # Operations on the empty string
         EQ("", "", "replace", "", "")
-
-        #EQ("A", "", "replace", "", "A")
-        # That was the correct result; this is the result we actually get
-        # now (for str, but not for unicode):
-        #EQ("", "", "replace", "", "A")
-
+        EQ("A", "", "replace", "", "A")
         EQ("", "", "replace", "A", "")
         EQ("", "", "replace", "A", "A")
         EQ("", "", "replace", "", "", 100)