Fix normalize_key to return appuid properly

[jacobhjkim]

                    for key in user:
                        key = key.strip(" \t\r\n\0\x0B.").lower()
                        key = cls.normalize_key(schema[counter],
                                                           str(key))
                        if schema[counter] != \
                                cls.Schema.MultiKeySchema.extern_id:
                            if isinstance(key, six.text_type):
                                key = key.encode('utf8')
                            key = hashlib.sha256(key).hexdigest()

In format_params, when a key's schema is Schema.MultiKeySchema.appuid, it will enter normalize_key function and return None. Thus, causing an error when you are trying to hash. Therefore, I propose to change the normalize_key function to return key_value when key's schema is Schema.MultiKeySchema.appuid

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[jacobhjkim]

Bug report related to this issue. https://developers.facebook.com/support/bugs/704470210011752/

[jingping2015]

Code already merge to GitHub, next release this will be get fixed. Thanks for helping fix this issue.