Checks that dict has only the appropriate keys for orient='split'.

def check_keys_split(self, decoded):
        """
        Checks that dict has only the appropriate keys for orient='split'.
        """
        bad_keys = set(decoded.keys()).difference(set(self._split_keys))
        if bad_keys:
            bad_keys = ", ".join(bad_keys)
            raise ValueError("JSON data had unexpected key(s): {bad_keys}"
                             .format(bad_keys=pprint_thing(bad_keys)))