Serialize the correct data into the named file object

def dump(self, msg, fn_):
        '''
        Serialize the correct data into the named file object
        '''
        if six.PY2:
            fn_.write(self.dumps(msg))
        else:
            # When using Python 3, write files in such a way
            # that the 'bytes' and 'str' types are distinguishable
            # by using "use_bin_type=True".
            fn_.write(self.dumps(msg, use_bin_type=True))
        fn_.close()