changes from issue http://code.google.com/p/pulp-or/issues/detail?id=45 LpProblem.deepcopy()

src/pulp/pulp.py

@@ -1125,7 +1125,7 @@ class LpProblem(object):
     def deepcopy(self):
         """Make a copy of self. Expressions are copied by value"""
         lpcopy = LpProblem(name = self.name, sense = self.sense)
-        if lpcopy.objective != None:
+        if lpcopy.objective is not None:
             lpcopy.objective = self.objective.copy()
         lpcopy.constraints = {}
         for k,v in self.constraints.iteritems():