SF bug #1012315: weakref.WeakValueDictionary should override .has_key()

* Check the found object for a None value during a contains/has_key
  lookup.  Perhaps it will help the OP who is likely suffering from an
  occassional GC or threading object deletion after self.data is checked.

* Complete the previous patch by removing the unnecessary indirection
  for weak dict iterators.  Makes the code cleaner and more readable.