TestResponse needs to handle multiple set-cookie headers

[simonw]

Seeing this test failure on #798:

_______________________ test_auth_token _______________________
app_client = <tests.fixtures.TestClient object at 0x11285c910>
    def test_auth_token(app_client):
        "The /-/auth-token endpoint sets the correct cookie"
        assert app_client.ds._root_token is not None
        path = "/-/auth-token?token={}".format(app_client.ds._root_token)
        response = app_client.get(path, allow_redirects=False,)
        assert 302 == response.status
        assert "/" == response.headers["Location"]
>       assert {"id": "root"} == app_client.ds.unsign(response.cookies["ds_actor"], "actor")
E       KeyError: 'ds_actor'
datasette/tests/test_auth.py:12: KeyError

It looks like that's happening because the ASGI middleware is adding another set-cookie header - but those two set-cookie headers are combined into one when the TestResponse is constructed:

datasette/tests/fixtures.py

Lines 113 to 127 in 0c064c5

	assert start["type"] == "http.response.start"
	headers = dict(
	[(k.decode("utf8"), v.decode("utf8")) for k, v in start["headers"]]
	)
	status = start["status"]
	# Now loop until we run out of response.body
	body = b""
	while True:
	message = await instance.receive_output(2)
	messages.append(message)
	assert message["type"] == "http.response.body"
	body += message["body"]
	if not message.get("more_body"):
	break
	response = TestResponse(status, headers, body)

[simonw]

This really needs a MultiValueDict ala Django: https://github.com/django/django/blob/24b82cd201e21060fbc02117dc16d1702877a1f3/django/utils/datastructures.py#L42

Turns out I have one of these in Datasette already - RequestParameters from 81be313

The name isn't quite right though.

[simonw]

I'm going to rename that MultiParams and use it in both places.