'Test the _write_header_footer() method footer only'

def test_write_header_footer_footer_only(self):

    self.worksheet.set_footer('&F')
    self.worksheet._write_header_footer()
    exp = '<headerFooter><oddFooter>&amp;F</oddFooter></headerFooter>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
