'Test the _write_border() method'

def test_write_border(self):

    xf_format = Format()
    xf_format.has_border = 1
    self.styles._write_border(xf_format)
    exp = '<border><left/><right/><top/><bottom/><diagonal/></border>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
