Uncaught AttributeError during decode

When a decode encounters a MessageField which has a non-dict value (e.g. "None" or "1"), an uncaught AttributeError is raised. The following is an associated stack trace:

```
File "apitools/base/py/encoding.py", line 110, in DictToMessage
return JsonToMessage(message_type, json.dumps(d))
File "apitools/base/py/encoding.py", line 104, in JsonToMessage
return _ProtoJsonApiTools.Get().decode_message(message_type, message)
File "apitools/base/py/encoding.py", line 290, in decode_message
message_type, result)
File "apitools/base/protorpclite/protojson.py", line 211, in decode_message
message = self.__decode_dictionary(message_type, dictionary)
File "apitools/base/protorpclite/protojson.py", line 284, in __decode_dictionary
for item in value]
File "apitools/base/py/encoding.py", line 312, in decode_field
field.message_type, json.dumps(value))
File "apitools/base/py/encoding.py", line 290, in decode_message
message_type, result)
File "apitools/base/protorpclite/protojson.py", line 211, in decode_message
message = self.__decode_dictionary(message_type, dictionary)
File "apitools/base/protorpclite/protojson.py", line 262, in __decode_dictionary
for key, value in six.iteritems(dictionary):
File "six/__init__.py", line 605, in iteritems
return d.iteritems(**kw)
```