Allow destructive ops during son iteration

and let python handle (identical to iter on list)

bson/son.py

@@ -120,13 +120,7 @@ class SON(dict):
     # efficient.
     # second level definitions support higher levels
     def __iter__(self):
-        """
-        Cannot remove nor add keys while iterating
-        """
-        key_len = len(self.__keys)
         for k in self.__keys:
-            if len(self.__keys) != key_len:
-                raise RuntimeError("son changed length during iteration")
             yield k
 
     def has_key(self, key):

test/test_son.py

@@ -163,11 +163,6 @@ class TestSON(unittest.TestCase):
         test_son = SON([(1, 100), (2, 200), (3, 300)])
         for ele in test_son:
             self.assertEqual(ele * 100, test_son[ele])
-        # test failure case
-        def break_iter():
-            for ele in test_son:
-                del test_son[ele]
-        self.assertRaises(RuntimeError, break_iter)
 
     def test_contains_has(self):
         """