Added a test for the envelope() GeoQuerySet method

                self.assertEqual(c.mpoly.sym_difference(geom), c.sym_difference)

                self.assertEqual(c.mpoly.union(geom), c.union)

    @skipUnless(getattr(connection.ops, 'envelope', False), 'Database does not support envelope operation')

    def test_envelope(self):

        "Testing the `envelope` GeoQuerySet method."

        countries = Country.objects.all().envelope()

        for country in countries:

            self.assertIsInstance(country.envelope, Polygon)

    @no_mysql

    @no_spatialite # SpatiaLite does not have an Extent function

    def test_extent(self):