Avoids a method invocation that introduces 6% improvement.

Author: cheqianh

amazon/ion/simple_types.py

@@ -713,7 +713,7 @@ def add_item(self, key, value):
         if key in self.__store:
             self.__store[key].append(value)
         else:
-            self.__setitem__(key, value)
+            self.__store[key] = [value]
 
     def get_all_values(self, key):
         """