Returns a list of all rows, with serialize_cell or self.get_cell_value called on the cells of
    each.

def serialize(self, serialize_cell=None):
    """Returns a list of all rows, with serialize_cell or self.get_cell_value called on the cells of
    each.
    """
    if serialize_cell is None:
      serialize_cell = self.get_cell_value
    return [[serialize_cell(cell) for cell in row] for row in self.rows]