'Test the _write_sheets() method'

def test_write_sheets(self):

    self.workbook.add_worksheet('Sheet2')
    self.workbook._write_sheets()
    exp = '<sheets><sheet    name="Sheet2"    sheetId="1"    r:id="rId1"/></sheets>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
