TestClient doesn't accept data with duplicate keys (SELECT multiple)

[Adam Gilman]

When constructing tests for forms with SELECT elements which allow multiple item selection, the test client doesn't accept multiple selections due to the fact it uses dict internally and results in a squashed single key

Failing test

    def test_post_multi_select(self):
        "POST SELECT multiple data to a view"
        post_data = QueryDict(mutable=True)
        post_data.update({'value':37})
        post_data.update({'value':37})
        response = self.client.post('/post_view/', post_data)

        # Check some response details
        self.assertEqual(response.status_code, 200)
        self.assertEqual(response.context['data'], ['37', '38'])
        self.assertEqual(response.templates[0].name, 'POST Template')
        self.assertContains(response, 'Data received')