'Test the _write_cell() method for formulas.'

def test_write_cell_formula01(self):

    cell_tuple = namedtuple('Formula', 'formula,    format,    value')
    cell = cell_tuple('A3+A5', None, 0)
    self.worksheet._write_cell(1, 2, cell)
    exp = '<c    r="C2"><f>A3+A5</f><v>0</v></c>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
