'Test the _check_sheetname() method'

def test_check_sheetname(self):

    got = self.workbook._check_sheetname('name')
    exp = 'name'
    self.assertEqual(got, exp)
    got = self.workbook._check_sheetname('Sheet1')
    exp = 'Sheet1'
    self.assertEqual(got, exp)
    got = self.workbook._check_sheetname(None)
    exp = 'Sheet3'
    self.assertEqual(got, exp)
