Applied to each column of every row returned by `rows`.

        Default behaviour is to escape special characters and identify any self.null_values.

def map_column(self, value):
        """
        Applied to each column of every row returned by `rows`.

        Default behaviour is to escape special characters and identify any self.null_values.
        """
        if value in self.null_values:
            return r'\\N'
        else:
            return default_escape(six.text_type(value))