'Test the _write_cell() method for numbers.'

def test_write_cell_number(self):

    cell_tuple = namedtuple('Number', 'number,    format')
    cell = cell_tuple(1, None)
    self.worksheet._write_cell(0, 0, cell)
    exp = '<c    r="A1"><v>1</v></c>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
