collections.defaultdict: Implement __contains__.

Otherwise, "in" operation uses list protocol (enumeration using
__getitem__, which adds a key/value pair to teh underlying dict)
and ends with memory overflow.

collections.defaultdict/collections/defaultdict.py

@@ -27,6 +27,9 @@ class defaultdict:
     def __delitem__(self, key):
         del self.d[key]
 
+    def __contains__(self, key):
+        return key in self.d
+
     def __missing__(self, key):
         if self.default_factory is None:
             raise KeyError(key)

collections.defaultdict/test_defaultdict.py

@@ -6,3 +6,5 @@ d[2] = 3
 assert d[2] == 3
 del d[1]
 assert d[1] == 42
+
+assert "foo" not in d